The Spine from Megaman, a little robot-spike-blade thingy that sits there and waits for Mario to walk on the same height as itself, and then gives chase.
I'm not sure how accurate this is to the Megaman original since I never saw it in action, but according to the person who requested it, it's good enough.
Graphics and palette included, instructions on how to adjust them at the top of the asm file.
Moderator's note: Indeed, it doesn't act like the original; the original sprite moved back and forth but sped up when the player was at the same horizontal position rather than being stationary until the player is both at the same horizontal position and on the ground. But it does at least seem to work.