Clean and Unclean Animals
Pythagoras taught the doctrine of the transmigration of the soul, but that it never entered into those animals which it is lawful to eat. Hence those animals which were lawful food for man were those into which the human soul never entered; but those into which the human soul entered were unclean or not fit for human food. This notion existed long before the time of Pythagoras, who learnt it in Egypt.
In the Old Testament, those animals which chew the cud and part the hoof were clean, and might be eaten. Hares and rabbits could not be eaten because (although they chew the cud) they do not part the hoof. Pigs and camels were unclean, because (although they part the hoof) they do not chew the cud. Birds of prey were accounted unclean. Fish with fins and scales were accounted fit food for man. (Lev. xi.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Clean and Unclean Animals from Infoplease: