Iwamodoki (岩もどき, or イワモドキ) is a Badnik from Sonic & Knuckles found lying under rocks in Lava Reef Zone.
They lurk in waiting until the player approaches, then flash briefly and explode, sending parts of themselves in various directions as projectiles. The Badnik cannot be destroyed by any means of attack, so it's best to just get out of the way of the projectiles, though they will bounce harmlessly off any shield currently active. After self-destructing, the animal inside will be set free, and a score value will appear as if the player had destroyed it. However, the points aren't actually added to the player's score. "Iwamodoki" means "pseudo-rock" in Japanese.
They return in Sonic Mania, again in Lava Reef Zone, where they'll use the same attack pattern. They, however, no longer release animals when they explode in this game.