Well, Thomas: going by the initial number of postings this is a sure fire hit.
Anyone truly interested in hidden or unknown animals has of course, merely to analyse any meat dish in Northern China.

And less, whimsically: the chap to search for cyber-chums is called Bernard Heuvelmans - he's been doing this (if he's still alive) for 40+ years.
The key is, I think, to distinguish very sharply between (a) animals that are certainly not unknown; but that are believed to be extinct. The quagga, thylacine etc.
(b) animals that reprsent clearly possible variants of known species: a new sort of dolphin, a very large or very small gorilla, a pigmy elephant
the above have at least some right to appeal to current knowledge or the fossil record
(c) the undefined and mysterious: yeti, kraken, bigfoot, the headless Paraguayan tree-panda etc.