After I uploaded Hijinks' ref, I figured I shoud explain my headcanon for Screwball.
Screwball's birth name is Serenity and she is the oldest daughter of Filthy Rich and Nurse Sweetheart. She changed her name to Screwball after she discovered her talent for pulling pranks and got her cutie mark.
When she was in middle school, she began smoking Poison Joke. Smoking the crushed up leaves of the blue plant can give one all sorts of crazy abilities (like the ability to fly), but it screws with the brain much like Ecstasy. Her parents try all sorts of things for many years to keep her clean, but nothing works and Rich and Sweetheart are forced to kick their daughter out when she begins to steal money from them.
Screwball is on her own for a long time, stealing and selling valuables in order to pay for her fix. She's staying in a homeless shelter when Discord arrives in Ponyville. Curious and unafraid, she follows the odd creature, who actually takes a liking to the silly little pony. The two play together while the Mane Six slowly make there way back home and Screwball asks if Discord could be her new daddy since her old one didn't want her. The draconequus, knowing how it feels to be abandoned, accepts the pony as his child.
After Discord is defeated, Screwball tries to attack Twilight, but is arrested. She remains in prison until Discord is brought back. He refuses to go with Celestia to Ponyville until his daughter is freed, so the princess releases her and allows her to stay in the castle as long as she promises to clean up.
So, yeah. There 'ya go.