No, not chihuahua, those are undisputedly real. Though some would argue that their nature is just as monstrous as an underworld demon of war.
The Chuchunya, living its cryptic life in the frozen land that is Sibiria, is a huge bipedal, hairy hominid. Reminiscent of the famous yeti, it mostly proves that the world is full of stories about hairy quasi-human things hiding in forests.
Steller's Sea Ape
This creature was first supposedly sighted by George Steller, a German zoologist, who sailed in the waters around Alaska in 1741. Steller reported a marine mammal of sorts curiously approaching the ship.
It reportedly had a large, densely-furred body, a whiskered, dog-resembling face, and the inquisitive and playful nature of a monkey, which prompted the creature's name. After watching the animal for about two hours, Stellar tried to shoot and capture it but missed. (Trust a human to see something cool and promptly look for a shotgun.) It is now speculated that the animal was a misidentified northern fur seal.
Isshii
Lurking in the water of Lake Ikeda in Japan, Isshii is the Japanese answer to the Loch Ness monster. Local myth says that the beast was once a mare whose foal was taken by a samurai. Desperate, she transformed herself into a huge, water dinosaur/reptile-like creature, and went in the water, often surfacing to look for her baby.
Those who claim to have seen it report a long (about 30 feet), large, black creature with two humps. The only video footage of it is often explained as a misinterpreted shot of big lake eels swimming near the surface.
Thunderbird
Okay, anything with a name this cool has got to be created by comic book writers, right? Well, not necessarily, because as far as we know, the indigenous population of North America was not into comics.
The indigenous myth says the bird could flap its wings and create thunder. Now, it's not like we have ancient accounts of those people to report to you, but it is theorized they based the legend on the local fossils of pterosaurs.
Mongolian Death Worm
Please excuse us as we slowly back away from the screen. Probably too much of a threat for any exterminator, this creature lives (supposedly) in the Gobi desert. What a wonderful reason to never visit.
A 1926 book written by American paleontologist Roy Chapman Andrews, includes accounts of Mongolians describing the monster as a desert poisonous sausage with no legs or head. If you think this sounds too much like a snake, you're probably right. Locals who claim to have seen the reptile were shown a tartar sand boa and confirmed that this was, in fact, the death worm.