For those who don’t play the game, mounts have three color variants. You get the first one with the mount, and the other two are unlocked per character as you level them up (currently level 7, then 9.)
 
The first color variant, seen above, is referencing Celestia. The colors aren’t quite a match, but the connection is clear once you know the other two colors. She also leaves a trail of particles as she runs, stars and hearts. The secondary color variation is blue, with a blue/purple mane, referencing Luna. She leaves a trail of moons. The third color is pink with pink/dark pink mane, referencing Cadance, and leaves a trail of hearts.
 
She was available as a bonus for making any real-money purchase in the game during the alpha to open beta. After launch, the bonus was changed to “Billy the Kid.” A goat using a slightly smaller model, in different shades of brown, tan and yellow for the color variations. Making a purchase after launch grabs you him as well.

You uncultured Horse, that’s a Goat! Ever see one before?

You are a sad, strange, little man and you have my pity…

>MOUNT ME!
 

 
X3

C’mon twilight, be respectful! He even has a cutie mark!

No, this is a weird pony-thing:  

Wonder Billie cracked me up the first time I saw it. It was such an obvious Blizzard-style MLP reference.
 
Funny thing is, there’s a couple characters that actually look right riding that thing.

Would be even funnier if it were an OC

Heh. Too bad it’s not one of the alt skins that make her (him?) look like Twilight.

I chuckled.