MLP FIM uses some fascinating creatures at times. Many we are already familiar with, but some of them are completely unheard of by anyone. I decided to do some research a little more in-depth and dug up a whole lot of stuff. I'm not going to write everything I find for the sake of simplicity, but I am supplying the links to my sources.
First off, changelings. They appear to be a simple brainchild of the writers initially so I was surprised when they turned up a legitimate article. According to mythology, changelings are the offspring of some creature such as a troll or fairy that is swapped with a human child. The motivation varies; sometimes it is out of spite and others for utilitarian purposes. For some creatures the love of a human is all they want.
An interesting Greek myth came up while researching the Ursas. The story is that Zeus wanted a woman named Callisto. Hera was jealous and turned her into a bear, and in this form she ran into her son Arcas. Arcas, not realizing that the bear is is mother, goes to shoot her. Zeus intervenes and turns Arcas into a bear and puts them into the heavans. Now, this is interesting because in the show the Ursa Major was depicted as Ursa Minor's mother. That correlates with the Greek myth.
I didn't think that Windigos' portrayal would be a far stretch from their actual mythology, but I was dead
wrong. They are of Algonquian origin, and are believed to be a cannibalistic spirit that possessed people, or alternatively a monster that people could transform into. Different groups described them differently, but all of them had the same basic principles. Windigos are always hungry because they grow proportionally to how much they eat, and thus they can never fill their stomachs. There is an association with winter and famine, but not as strongly as portrayed in MLP FIM. I wonder if the Windigos in the show eat the ponies they kill. Hmmm.... yep. that's my headcannon.
Discord the draconequus is very interesting. It turns out he is fairly original. The species name is composed of the latin words for horse and dragon. Technically he is a chimera due to his body being composed of different animal parts. The sight of a chimera was believed to bring bad omen such as natural disasters. Another obscure detail I found in my research was a reference to a battle in Greek mythology. A man named Bellerophon rode a Pegasus into battle with it and he won because the Pegasus was quick enough to dodge the fiery breath. When against Discord the pegasi were the only two he had any trouble with. Fluttershy couldn't be won over so he had to physically touch her for the spell to work. Rainbow Dash wasn't turned against Loyalty, she just had her priorities changed. Discord was partially beaten by the pegasi just as the chimera was beaten by the Pegasus.
Something to note is that the chimera, the hydra, and Cerberus are all siblings in mythology. the hydra lives in a lake and guards the entrance to the Underworld and Cerberus guards the gates.
The cockatrice is pretty cut-and-dry. An intriguing detail is that the cockatrice is comparable to the Medusa, and when Perseus killed her in myth, a pegasus
was born from her blood. Okay, another headcannon-Fluttershy is the descendant of Medusa, and that is why she can use the stare.
The ahuizotl of Aztec origination is an odd being. It is a creature of rubbery smooth skin and possessed a hand on it's tail that it used to drown prey. To entice people to it's lair it would sometimes use a child's' cry and, once the person was dead, it would eat him. The nails, eyes, and teeth were the most favored parts, though human flesh in general was the ahuizotl's favorite meat. The 8th Aztec king named himself after the ahuizotl to represent his greatness. The king was known as the greatest military leader of their time, so that could be why in the Daring Do book he leads an army of cats. They also had a warrior sect called the Jaguars.
The rest are pretty standard, many being plays on words, such as Timber Wolves, Quarry (Moray) eels, and fruit bats. Others are fairly well known like phoenixes and various dragon-like creatures.
If anyone see's something that is wrong or has something to add that isn't too in-depth, you can let me know in the comments and leave a link so I can put it here. There is just so much out there that to put everything here would be like writing a book about mythology.
If you want more in-depth information check out my highly
Here are my sources:[link]
MLP Wikia-Some kind of Discord forum thingy[link]