A person who believes that they are, say, a crocodile or a werewolf may be suffering from rare a psychiatric condition known as Lycanthropy. As T.A.Fahy (MB BCh BAO) noted in his extensive roundup of the subject Lycanthropy: a review (published in the Journal of the Royal Society of Medicine Volume 82 January 1989) the syndrome can be traced back at least as far as ancient Rome, when commonly personified animals tended to be wolf-like or goat-like.
Nowadays wolves are still prevalent, but the most common current illusory animals also include dogs and cats. And Dr. Fahy also came across “…one case who identified himself as a gerbil…”. Presumably an extremely large one. It should be noted though that not all lycanthropic manifestations are necessarily genuine, as Fahy pointed out –
“Two patients, one who claimed to be a Bengal Tiger and one who hopped around a ward like a rabbit, admitted that their behaviour was under voluntary control after confrontation by their psychiatrists, and were judged to have a factitious element to their accompanying psychiatric disorder.”
Note : The illustration shows part of an engraving by Hendrik Goltzius (1558-1617) in which the Greek Mythological character Lycaon, after whom the syndrome is named, is transformed into a wolf by Zeus. [click to view in full]