Smarty Pants was Twilight Sparkle's childhood doll. In "Lesson Zero", Twilight tried to use the doll to start a fight among the Cutie Mark Crusaders, so that she could "teach" them to share so she'd have something to report to Princess Celestia. Apple Bloom, Sweetie Belle, and Scootaloo didn't want the doll, to Twilight used a "want it, need it" spell on Smarty Pants, causing anyone who looked at it to gain the desire to possess it to the exclusion of all else. Celestia removed the enchantment. Big McIntosh took the doll. He still had it by the time of the episode "Ponyville Confidential", as a picture of him with Smarty Pants appeared in the Gabby Gum's column of the Foal Free Press.
Smarty Pants is included, unlabeled, in the Pony Lesson Set.