If you've never heard of Sleipnir, buckle down.

Sleipnir is a horse from Norse mythology, and he is basically the best horse ever. He had 8 legs and was Odin's personal steed. He was stronger and faster than any horse out there. But the best part about him is his origin story.

Back when all the gods were out building their god-city Midgard, a giant came and offered to help them build parts of it. In exchange, he wanted the sun, the moon, and the goddess Freyja who was in charge of sex and stuff. All the gods were kinda not okay with this, so they put a bunch of restrictions on the dude and said that he had to work alone and finish in 3 seasons. They basically made it impossible for him to do what he was promising, and expected to get 90% of a bad-ass wall for their city for free.

The giant said okay, but only as long as he could have his horse help him. The gods were against that at first, since they really wanted to hamstring him any way they could, but then Loki jumps in and says "Sure, why not!" So he gets to work.

Well, it turns out that his horse was fucking awesome. The horse was basically doing all the work. So when it becomes clear that the giant is going to finish in time, all the other gods get pissed off at Loki for letting the guy use the horse in the first place. And also because blaming Loki is basically what everyone does all the time anyway.

"Fix it Loki!" they all say.
"Okay." says Loki.

That night, the giant and his horse go out to get some more stone for the wall. Unbeknownst to them Loki is hiding out. Disguised. As a mare. Loki steps out, gives the horse the bedroom eyes, and the overworked stallion breaks his line and runs off after Loki.

Without his horse, the giant can't finish the wall. The gods get to keep the sun, the moon, and the goddess of banging. Also, Thor ends up killing him for some reason. And Loki disappears for several months...

...and when he comes back, he has an 8-legged foal with him. Sleipnir! Turns out, Loki got knocked up by the horse. So Sleipnir is Loki's kid, and Loki is the mother.

TL:DR Loki fucked a horse and got pregnant.