Of course, D&D is not the first place to do so, but one thing I've always liked about D&D was that it took monsters that were unique in Classical Mythology and made them into non-unique monster types. There was only one Hydra, one Medusa (although three Gorgons), one Chimera, one Minotaur. In D&D, we get hydras, medusae, chimerae, minotaurs, etc.
Of course, Greco-Roman mythology has some 'monster races' such as centaurs and satyrs and mermaids, too.
But other mythologies didn't always get the same treatment. There are plenty of monsters in D&D taken from Norse Myth, but mostly they're the ones that are 'monster races' in the myths. Elves, dwarves, trolls, frost giants, fire giants, and the like. But there are plenty of unique monsters from the myths that could easily become monster races in D&D. As far as I know, though, they've never been written up (unless they were in some old issues of Dragon or something).
So I'm gonna try my hand at stats for some of them over the next few days. We'll see how it goes. Let's start with Sleipnir. Odin's mount in the myths, it can become the sort of creature adventurers seek out as a mount when they need to travel somewhere fast (like the Firesteeds of Krull, which could also use stats), or between worlds.
Armor Class: 4 (16)
Hit Dice: 6*
Move: 360 (120)
Attacks: 4 hooves
Damage: 1d6 each
No. Appearing: 1d3 (3d6)
Save As: Fighter 6
Treasure Type: I
Sleipnirs are large horse-like creatures, slightly larger than normal war horses. Their most unusual feature, sometimes not even noticed at first, is that they have eight legs. They are semi-intelligent and once they bond with a rider, by choice or by training, they are fiercely loyal to that rider for life. In combat, sleipnirs rear up and strike with all four of their front legs. Sleipnirs are prized for their great speed, but also, once per day, a sleipnir and its rider may planeshift to an adjacent plane of existence.