Twilight Bockle is a purple chicken who lives in a small coop outside the treebrary. Her feathers are oddly in the same exact coloration as Twilight Sparkle, right down to a cutie-mark style pattern on one of her sides.
Take Twilight, make her a chicken without magic. There ya go! Sure, she knows she's not a pony, and that she's a chicken, but deep down, she really thinks she's supposed to be a brainy bird.
She began life as a shadow magic created bunny, part of a larger herd of shadow bunnies meant to distract the citizens of Ponyville from something more important going on in town. When the ponies began to figure out something was amiss, Denise used a dispel spell that had subconscious polymorphic side effects on some of the bunnies. Twilight Bockle was the end result. Named and adopted by the real Twilight, she spends most of her day being a chicken.
Her only skills are that of a normal Equestrian chicken.