From Sonic Retro
Iwamodoki (岩もどき) is a badnik from Sonic & Knuckles found lying under rocks in Lava Reef Zone. They lurk in waiting until Sonic approaches, then flash briefly and explode, sending parts of themselves in various directions as projectiles. The Badnik cannot be destroyed by Sonic, 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 Flicky 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.