<unk> is the American medical slang for arriving at a surprising, often exotic, medical diagnosis when a more commonplace explanation is more likely. It is shorthand for the aphorism coined in the late 1940s by Theodore Woodward, professor at the University of Maryland School of Medicine, who instructed his medical interns: "When you hear hoofbeats behind you, don't expect to see a<unk> . Since horses are common in Maryland while<unk>  are relatively rare, logically one could confidently guess that an animal making hoofbeats is probably a horse. By 1960, the aphorism was widely known in medical circles. Medical novices are predisposed to make rare diagnoses because of (a) the availability heuristic ("events more easily remembered are judged more probable") and (b) the phenomenon first enunciated in Rhetorica ad Herennium (circa 85 BC), "the striking and the novel stay longer in the mind." Thus, the aphorism is an important caution against these biases when teaching medical students to weigh medical evidence. Diagnosticians have noted, however, that<unk> "-type diagnoses must nonetheless be held in mind until the evidence conclusively rules them out: Comparable slang for an obscure and rare diagnosis in medicine is fascinoma.
Zebra 
Wellbee
Primodos
Chordee
Corn 
Botfly
Charley horse
Leech