Often specifically a donkey. Note that this is a noun form of the Greek -phagos (eater) root, instead of the more common -phagous ajectival form. The plural is cardophagi. As for the cardo- part, well, you can guess that that's also Greek for thistle. As for when it was coined, dictionaries aren't saying, but I suspect it of being an bit of 18th century classicism -- but that's really just a guess. It's entirely possibly it was coined by or in response to A.A. Milne to describe Eyeore.
The cardophagus looked up at Pooh and said, "Oh, it's you. It must be one of those days."