Flying reptiles? Got 'em. Some from Photoshop, some from the brief paranormal TV show Freakylinks, some from...well, somewhere. Sharon Hill mentioned it here along with other "paranormal" hoaxes. The life-sized pterodactyl made for Freakylinks has ended up, as it should, in the International Cryptozoology Museum. (Scroll down on this ICM page for some other hoax creatures, the most famous being Barnum's Feejee Mermaid. )
Most recently, we have the "giant giant giant" squid from California, blamed on mutating radiation. (I've sometimes wondered if all of California could be blamed on mutating radiation.) There is, however, no endless supply of calamari to draw from. See Snopes for an explanation of how a dead whale photograph and a dead squid photograph were morphed into a Ray Harryhausen-level monster.
Photo copyright unknown, but when you publish a hoax, I'm not likely to ask you for permission.
And, finally, we have not one but two "dead bigfoot" stories making the rounds. The odds that Rick Dyer has a dead sasquatch (you remember Dyer, right? He showed off a fake at a press conference years ago), or that Justin Smeja killed two sasquatches and collected absolutely no evidence are approximately equal to the odds my dog has a live squatch cornered in my living room.
Hoax animals can be a lot of fun to create and fun to talk about. Sometimes there are other agendas involved, derived from religious literalism or from "ancient alien" beliefs.
These come into play in some of the endless "ancient human giant" hoaxes, which use real skeletons shown in forced perspective or blown up through Photoshop, or in one case a giant finger that could be a tree root for all I can tell. My response to those is always "Try this homework assignment: Design a workable knee joint for a 20-foot-tall biped in Earth's gravity. HINT: This is not possible."
King Kong, or King PhotoShop?
Giving history the finger
These are related to the endless repeats of old U.S. newspaper clippings about human giants discovered but of course covered up by the scientific establishment. Boy, do those get tiring. Try to imagine a scientist with a gigantic cranium on his desk says, "Despite this being irrefutable evidence anyone can come and see for themselves, I'll forego the Nobel Prize so as to not rock the boat."
Karl Shuker collects some eye-popping hoaxes here, including terrific "rogue taxidermy" animals that would be genuinely startling/puzzling if you didn't know the context.
The Internet, of course, has made hoaxing a million times more fun for the hoaxers and a million times (I'm presuming there is some scientific formula for calculating this, but math is not my thing) more exasperating for real scientists who have to waster time pointing out what's going on. But we'll always have hoaxes. If we find animals on other planets, the hoaxers will be right behind the scientists. It seems to be human nature.