Her name is Surprise and she is a pony from another (I'm sorry i don't know which off the top of my head) generation of MLP. she is a white Pegasus with blonde mane and tail and three (3) balloons as her cutie mark. she is the inspiration for pinkie pie and because of that people like to have them together like this.

While not in the show she is very popular, like Derpy and Octavia, all the same