Cryptids are a fascinating branch of the monster family. Cryptids are unrecognised, often mythical plants or animals, though some cryptids go on to become recognised species (the Okapi was once a rumour, later to be proven true). Here is a Wikipedia list of cryptids, everything from extinct creatures to out and out hoaxes.
The fascinating ones are of course the myths, the true monsters. There are a few themes; human/animal hybrids, water dwellers and beasts in the forest. These are respectively ancient sublimations of the outlaw, the foreigner and awe in the face of nature. Water and the forest serve in morality tales: drowning in water = sorrow, losing one’s way in the forest = madness.
Cryptids are part of rural cultures around the globe. Nature has been largely expunged from daily life core capitalist countries, especially the daily life of the upper classes. When nature reappears it often seems like a violent invasion, a natural disaster or a premonition. There’s an interesting chapter in Mike Davis’s Ecology of Fear. Strange things have begun to happen in suburban California