Today's monsters are more of a sort of Medieval joke, a social satire, than anything anyone ever believed actually existed. They are the Bicorne and the Chichevache, and they are generally spoken of together, as a pair of opposites.
The Chichevache (or "scrawny cow"; also known in French as the Chichifache, or "thin or ugly face"; also called the Thin-gut) is described as an extremely thin cow with a miserable, ugly human face. It's state of malnourishment and misery was said to be caused by it's diet: it fed only on faithful and obedient wives, said to be in short supply.
The Bicorne (or Bycorne, Bicorn, and sometimes the Burrowing Bicorne; also called the Bulchin) in contrast looks like a grossly plump panther with a pleased expression. This is because the Bicorne feeds on hen-pecked, subservient husbands, of which it can find a great abundance. The Bicorne is said to have two horns (one suspects the ubiquitous 'cuckold horns' joke so beloved of the Middle Ages and Renaissance) that it drops when captured.
As obscure as these creatures are, they have a presence in literature: Chaucer in "The Clerk's Tale" cautions wives not to be too good lest the Chichevache devour them, and Bicorne horns are mentioned in the Harry Potter books as an ingredient in Polyjuice potion.