Unclean animal

In some religions, an unclean animal is an animal whose consumption or handling is taboo. According to these religions, persons who handle such animals may need to ritually purify themselves to get rid of their uncleanliness.
Judaism.
In Judaism, the concept of "impure animals", plays a prominent role in the Kashrut, the part of Jewish law that specifies which foods are allowed ("kosher") or forbidden to Jews. These laws are based upon the Books of Leviticus and Deuteronomy of the Torah and in the extensive body of rabbinical commentaries (the Talmud). 
The concept of unclean animals is also mentioned in the Book of Genesis, when Noah is instructed to bring into the Ark all sorts "of pure beasts, and of beasts that are impure, and of fowls, and of every thing that creepeth upon the earth".
In the Torah, some animals are explicitly named as pure or impure, while others are classified by anatomical characteristics or other criteria. In some cases, there is some doubt as to the precise meaning of the Biblical Hebrew animal name.
According to Jewish dietary laws, to be "pure" an animal must also be free from certain defects, and must be slaughtered and cleaned according to specific regulations ("Shechita"). 
Any product of an impure or improperly slaughtered animal is also non-kosher. Animal gelatin, for example, has been avoided, although recently kosher gelatin (from cows or from fish prepared according to kosher regulations) has become available. (The status of shellac is controversial.) The prohibitions also extend to certain parts of pure animals, such as blood, certain fat tissues, and the sciatic nerves. 
Finally, it is forbidden to cook the meat of an animal in the milk or dairy product of that same animal, which has in turn led to the traditional practice of using separate complete sets of kitchen utensils for meat and dairy so as to totally ensure this rule is not broken.
Classification of animals.
The Torah does not classify animals under modern scientific categories of mammals, fish, reptiles and birds. Rather, the religious categories are land-dwelling animals (land mammals, flightless birds, and land reptiles, etc.), flying animals (birds, insects, flying mammals such as bats), and given that each of these religious categories of animals includes species of at least two or more of each scientific categories of animals, there is no general "kashrut" rules relating per se to mammals, birds, reptiles, or fish. However, rules for each of these classes of animals can be extrapolated from the biblical requirements.
Mammals.
According to the Torah, land-dwelling animals that "both" chew the cud (ruminate) and have cloven hooves, are kosher. 
By these requirements, any land-dwelling animal that is kosher can only possibly be a mammal, but even then, permitted are only those mammals that are placentals and strictly herbivorous (not omnivores nor carnivores) that both ruminate and also have cloven hooves, such as bovines (cattle/cows, bison, buffalos, yak, etc.), sheep, goats, deer, antelope, and technically, also giraffes.
Although the giraffe falls under the kosher category by its characteristics, it does not have a masorah (tradition) for its consumption by any Jewish community. Though it is commonly believed that it is not known where on a giraffes neck "shechita" (ritual slaughter) can be performed, this is incorrect as the "shechita" can be performed anywhere on the neck.
All other mammals, land-dwelling or otherwise, are forbidden by the Torah, including "crawling creatures" such as mice, and flying mammals such as the various species of bats. 
Water-bound mammals, such as whales, dolphins, orcas, and dugongs, are also not kosher as they do not have the characteristics required of kosher water-bound creatures which must have both fins and scales.
Those land-dwelling mammals that have only one of the two characteristics of kosher land-dwellers (only ruminant or only cloven hooved) are impure and cannot be consumed. 
By default, therefore, not only are most land-dwelling mammals not kosher, but all land-dwelling non-mammals are also not kosher, including reptiles, amphibians, molluscs (including snails), etc.
Among mammals that Leviticus cites explicitly as an example of unclean is the camel, because it ruminates but does not have a cloven hoof; the hyrax and the hare are also explicitly given as an example of being excluded as kosher on the same grounds. Quintessentially, the Torah explicitly declares the pig unclean, because it has cloven hooves but does not ruminate.
It is of interest to note that Australia is the only continent that has no kosher native mammals, nor kosher native birds. Thus, prior to European settlement, there were no kosher land-dwelling or flying animals in Australia. Though the kangaroo chews its cud, for instance, it does not have hooves, and is therefore not kosher. No mammals that are marsupial or monotreme are kosher.
Fish.
According to , anything that comes from the water ("in the seas, and in the rivers") that has both fins and scales may be eaten. By those requirements, kosher water creatures can only possibly be fish, but even then, permitted are only those fish that have both fins and scales.
All other non-fish water creatures are, by default, also not kosher, including amphibians, crustaceans, molluscs, water-bound mammals, water-bound reptiles, etc.
While there is nothing specifically mentioned in Jewish "halakha" requiring kosher fish having an endoskeleton ("inner skeleton") and gills (as opposed to lungs), every true fish that has both scales and fins by default also possesses an endoskeleton and gills. 
Any sea creature that lacks gills and can only breathe oxygen from air through lungs, or has an exoskeleton instead of and endoskeleton, is by default not kosher.
The definition of scales does not include the shells of prawns and shrimp, which are in fact the exoskeleton ("outer skeleton") of these animals, in the same manner as the shells of lobsters or crabs. Even if these shells were to be misidentified as scales, these creatures would still not be kosher as they lack fins.
While not every fish that has fins will have scales, every true fish that has true fish scales by default also has fins.
Birds.
The Torah names only a few birds that may not be eaten; those not in the list are presumed to be kosher. However, the precise identity of the unclean birds is a matter of contention in traditional Jewish texts. It is therefore common to eat only birds with a clear "masorah" (tradition) of being kosher in at least one Jewish community, such as domestic fowl.
Leviticus 11 lists, among other things, the specifically non-kosher birds. The Hebrew names listed have been translated as follows:
Bats are also mentioned (though biologically, these are mammals rather than birds).
Insects.
The Torah allows eating certain kinds of "winged swarming things" (e.g. insects) while prohibiting others. However, due to uncertainty about the Hebrew insect names, rabbis today recommend that all insects be considered unclean. 
An exception is made for certain locusts ("Schistocerca gregaria"), which are traditionally considered kosher by some Yemenite Jewish communities.
 details which insects are not to be eaten, and due to the wording all insects are considered impure to avoid mistaken consumption.
Bees' honey is considered kosher because the honey is not a product made of bees.
Explicit list.
The following animals are considered to be impure according to and , based on Rashi's identification:
Reasons.
Some scholars have conjectured that the Jewish concept of "unclean animals" arose out of public health concerns by community leaders, since, in the conditions of the times, some of those animals are indeed more likely to cause food poisoning or transmit diseases to people who consume them.
British anthropologist Mary Douglas proposed that the "unclean" label had philosophical grounds, namely it was cast on foods that did not seem to fall neatly into any symbolic category. The pig, for example, was seen as an "ambiguous" creature, because it had cloven hoof like cattle, but did not chew cud.
Christianity.
In the very early days of Christianity it was debated if converts ought to follow Jewish customs (including circumcision and dietary laws) or not. 
According to the account of the Council of Jerusalem in , a compromise was reached between those who wanted full compliance and those who favored a more liberal view.
It was agreed that the converted Gentiles would have to bear "no greater burden than these necessary things: that ye abstain from meats offered to idols, and from blood, and from things strangled, and from fornication".
Jesus is quoted in as saying "There is nothing from without a man, that entering into him can defile him ... whatsoever thing from without entereth into the man, it cannot defile him; because it entereth not into his heart, but into the belly"; and in . "Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man." These statements are often cited for support of the view that practicing Christianity does not include dietary restrictions.
Supporters of the liberal view also point to Peter's vision reported in and in which God invited him to "kill and eat" from the animals in the "great sheet" containing "all manner of fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air". 
They also draw support from the writings of apostles Timothy (, "For every creature of God is good, and nothing to be refused, if it be received with thanksgiving; for it is sanctified by the word of God and prayer") and Paul (, "Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days").
While the majority of Christians agree that the dietary restrictions of the Old Testament were lifted with Christ's New Covenant, a view known as supersessionism, there are Torah-submissive Christians who believe that they should still be observed. Supporters of this view may argue, for example, that in the Old Testament, Daniel spoke of unclean food and drink as "defiling one's body" , and that in the New Testament one's body is said to be the "temple of God", and "If anyone defiles the temple of God, God will destroy him".
Some read Jesus's reply to questioning by the Pharisees in and as implying that his statements about "which goeth into the mouth" ( and ) referred to the question of hand washing, rather than clean and unclean foods.
Others also argue that the dietary restrictions predate Leviticus, and that Paul in was referring to the ceremonial feast days such as the Feast of Unleavened Bread and not clean and unclean foods.
Others argue that the liberal view would imply the acceptance even of alcohol, tobacco, rats and roaches as "clean food"; and that God never declares something an abomination and then changes His mind.
Supporters of the stricter view have also disputed the interpretation of Peter's vision , claiming that God was merely instructing him not to refer to gentiles as "unclean" since salvation had been extended to them. This is expressly stated by Peter later in the chapter at Acts 10:28 ("but God hath shewed me that I should not call any man common or unclean.") 
In Acts 10:14 Peter makes a distinction between "common" (Greek κοινόν) and "unclean" (Greek ακάθαρτον) to which God replies in the next verse "What God hath cleansed, that call not thou common [κοίνου]".
One modern example of a Torah-submissive group is the Seventh-day Adventist Church whose co-founder Ellen G. White was a proponent of vegetarianism. Many Seventh-day Adventists avoid meat for health reasons, though vegetarianism is not a requirement. Members of the United Church of God as well as other Sabbath-keeping Christian Churches also believe in abstaining from unclean meats.
Roman Catholic traditions.
In the Roman Catholic Church, it was forbidden to eat meat (defined as the flesh of any warm-blooded animal) on Friday, but as a penance to commemorate Christ's death rather than for meats being regarded as "unclean". 
In the 1983 Code of Canon Law, the current code of canon law for the Roman Catholic Church, the Friday abstinence from meat is prescribed for "those who have completed their fourteenth year of age". Once a person has begun his or her sixtieth year, the abstinence is no longer obligatory. 
Canon 1253 allows each particular conference of bishops to "determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast". 
The current disciplinary norms from a document produced by the United States Conference of Catholic Bishops, titled "Pastoral Statement On Penance And Abstinence", remove the penalty of sin for Roman Catholics in the United States who choose not to abstain from meat, while the document "give[s] first place to abstinence from flesh meat". 
Roman Catholics in the United States are, therefore, free to substitute some form of penance on Fridays of the whole year, while the Lenten abstinence remain obligatory.
Seventh-day Adventist.
Adventists are known for presenting a "health message" that advocates vegetarianism and expects adherence to the kosher laws, particularly the consumption of kosher foods described in Leviticus 11, meaning abstinence from pork, shellfish, and other animals proscribed as "unclean".
Islam.
In Islam several animals are considered unclean and their consumption is sinful ("harām"), except in case of necessity; while others are permitted ("halāl"), as long as they are slaughtered in the proper manner and with blessings given to God.
The Qur'an expressly forbids consumption of "the flesh of swine" There are no other "impure animals" explicitly named in the Qur'an. If someone converts to Islam, Allah "allows them as lawful what is good and prohibits them from what is bad; he releases them from their heavy burdens and from the yokes that were upon them".
For other animals, great importance is given to the manner of its death: forbidden are blood and carrion ("dead meat"), and any animal that has been "killed by strangling, or by a violent blow, or by a headlong fall, or by being gored to death". Also forbidden is any animal that has been eaten by a wild animal, unless the person is able to slaughter it before it dies.
Finally, the Qur'an forbids food which has been invoked by a name other than Allah, which has been sacrificed on stone altars, or has been subjected to the pagan practice of raffling with arrows. Food slaughtered by an idolater is forbidden, but food that is acceptable to Jews and Christians is allowed to Muslims as well.
Dogs.
According to the majority of Sunni scholars, dogs can be owned by farmers, hunters, and shepherds for the purpose of hunting and guarding and the Qur'an states that it is permissible to eat what trained dogs catch. Among the Bedouin, the saluki dogs are cherished as companions and allowed in the tents.
Nevertheless, many Islamic teachers state dogs should be considered unclean and that Muslims licked by them must perform ritual purification. According to a Sunni Islam Hadith, a plate that a dog has used for feeding must be washed seven times, including once with clean sand mixed with the water, before a person may eat from it.