The Nid from Donkey Kong Country 3, as seen in the Springin' Spiders stage. If the extra bit is clear, it will be yellow and hop up and down in place. If the extra bit is set, it will be purple and will move towards Mario while hopping. Mario can stand on top of it.
The sprite comes with two configuration files. One of them will let Mario kill the sprite with a spin-jump, while the other will make him stand on the sprite normally.
Check the ASM file for customization options.
Tester's Note: Mario falls for a couple of frames when the sprite approaches the ground. It doesn't seem to affect gameplay, but it looks a little odd.