Thwimp

Thwimps are tiny versions of the large, stone enemies, Thwomps, that first appeared in the SNES version of Super Mario World.

Super Mario World
Thwimps appeared in both the SNES and the Game Boy versions of Super Mario World where they were enemies and hopped back and forth in large arcs trying to land on, and kill, Mario or Luigi. Not unlike their larger cousins, the thwomps, they cannot be destroyed and can kill the player very easily so the player has to avoid them. They usually appear in groups of two and cause obstacles in small hallways.

Mario vs. Donkey Kong
Thwimps also made a few other appearances in later games, such as Mario vs. Donkey Kong where they were enemies that appeared in the Spooky House world of the game. The thwimps once again tried to defeat Mario by landing on him and are invincible but once again Mario must avoid them to defeat Donkey Kong and win the level.

Super Mario Bros. 3: Super Mario Advance 4
Thwimps are also found in the E-Reader level "Swinging Bars of Doom".

Trivia

 * Thwimp's name is a portmanteau of "Thwomp" and "wimp".