Mare Do Well is a masked hero who appeared on the animated TV series My Little Pony: Friendship is Magic in the episode "The Mysterious Mare Do Well".
Mare Do Well can sense when something is about to happen (i.e. falling rubble). She can also wield powerful magic, has enhanced strength, and has wings.
It was revealed at the end of the episode "The Mysterious Mare Do Well" that Mare Do Well was actually Twilight Sparkle, Fluttershy, Pinkie Pie and Applejack working in shifts, using a costume designed by Rarity, as part of a plan to teach Rainbow Dash a lesson.