In Lauren Faust's original pitch for Friendship is Magic, she envisioned Pinkie Pie as a pegasus with a penchant for flying upside down and surprising other ponies by sneaking up on them. Faust based Pinkie's personality and design on her own interpretation of a white G1 pegasus pony named Surprise. Surprise primarily shows up in fan-art, comics, and an Ask Pony blog, but she has also made appearances in fan-fictions like Surprise, Surprise.
Surprise is generally portrayed as Pinkie Pie with wings: cheery, boundless in energy, a lover of parties, and usually with a similar balloon cutie mark.
The Ask blog Surprise is a member of the Wonderbolts and frequently restyles her mane at the request of readers. She also fields a wide range of random questions without batting an eyelid, although she does deadpan from time to time.