I do love a debunking, and Victor Mair provides a good one at the Log:
If you do a web search for “Hsigo”, you will find thousands of references and hundreds of images. I won’t give specific references, because they’re all complete and utter nonsense, but you can read detailed descriptions of these fake, mythical Chinese monkeys — including pseudo-learned discussions of their name — in works like the following: Erudite Tales, Creepy Hollows Encyclopedia, Mythical Creatures Guide, Encyclo, Societas Magic, Monstropedia, etc., etc. Hsigo are supposedly flying monkeys with bird-like wings, the tail of a dog, and a human face. […] It all started with a typo.
Do read the whole thing; it’s quite entertaining.