Thwimp

Thwimps are significantly smaller variants of the large, stone enemies, Thwomps, that first appeared in Super Mario World. Their name is a portmanteau of "Thwomp" and "wimp", making reference to their diminutive size.

Super Mario World
Thwimps appear in both the SNES and the Game Boy Advance versions of Super Mario World where they are enemies and hop back and forth in large arcs trying to land on Mario or Luigi. They first appear in #2 Morton's Castle. They usually appear in groups of two and cause obstacles in small hallways. In #3 Lemmy's Castle, the Thwimps created by Magikoopas can be destroyed by shells. However, the Thwimps are only vulnerable for a few seconds, after which the shell will have no effect.

Super Mario Advance 4: Super Mario Bros. 3
Thwimps are also found in the e-Reader level "Swinging Bars of Doom" in Super Mario Advance 4: Super Mario Bros. 3. They can now be defeated by Statue Mario, Invincible Mario, or by Hammer Mario's hammers.

New Super Mario Bros. U
After eight years of absence, Thwimps reappear in New Super Mario Bros. U. They act the same as they did in Super Mario World and also have the same overall appearance. They can be defeated by hitting a ? Block while the Thwimp is above one, or by carrying a frozen enemy under one. Additionally, blue cracked blocks can be spotted on the place Thwimps stomp. They only appear in Wendy's Shifting Castle. They sometimes spin while jumping.

The Thwimp is one of the very few enemies that do not reappear in New Super Luigi U.

Mario vs. Donkey Kong
Thwimps also make a few other appearances in later games, such as Mario vs. Donkey Kong where they are enemies that appear in the Spooky House world of the game. The Thwimps try 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.

Trivia

 * In the Super Mario Mash-Up Pack in Minecraft, Slimeballs are replaced by Thwimps.
 * Cycloptic versions of Thwimps appear in side-scrolling segments of the Game Boy and Game Boy Color games in the Legend of Zelda series. However, they act more like Thwomps, and the English version even refers to them simply as Thwomps.