World 4-3 (Super Mario Bros. 2)
From the Super Mario Wiki
Pink Birdo is seen as a seemingly normal enemy early in level, and reappears after being defeated whenever the player reenters the room it is in. This allows it to be defeated an infinite number of times for a massive amount of points. The actual purpose of this out-of-place Birdo is for the player to ride on one of its eggs across the otherwise impassable ocean to reach the main dungeon where Fryguy's lair is located.