Humans.

Truth or myth? Ancient lost civilization or just an old pony's tale?

No pony has ever seen one, and most haven't even heard of them. But Lyra knows that these wonderful creatures are more than just old legends, and she's going to find out... and possibly drive her roommate insane in the process.

Prince Blueblood is banned from frolicking within the city limits of Canterlot. In an act of defiance, he strikes out to make a name for himself in Ponyville. Meanwhile, a lovelorn Big Macintosh pines for Twilight Sparkle, who only sees him as a big, dumb farmpony. Worlds collide when, with the help of some stolen Zebra magic, the poor, unfortunate souls switch bodies.
Now, Blueblood runs amok through the mares of Ponyville while Big Macintosh learns that being Twilight's Prince Charming is harder than it looks.The sleepy town of Ponyville is in for a rude awakening!