Unipedalism

The term uniped (from Latin "uni-" "one" and "ped-" "foot") refers to a person or creature with only one foot and one leg, as contrasted with a biped (two legs) and a quadruped (four legs). Moving using only one leg is known as unipedal movement. Many bivalvia and nearly all gastropoda molluscs have evolved only one foot. Through accidents (i.e. amputation) or birth abnormalities it is also possible for an animal or a human being to end up with only a single leg.
In fiction and mythology.
One major study of mythological unipeds is Teresa Pàroli (2009): "How many are the unipeds' feet? Their tracks in texts and sources", in "Analecta Septentrionalia: Beiträge zur nordgermanischen Kultur- und Literaturgeschichte", ed. by Wilhelm Heizmann, Klaus Böldl and Heinrich Beck (Berlin/London/New York: De Gruyter), pp. 281–327.