From Sonic Retro
The Hyudoro is a ghost enemy found in the pyramids of Sandopolis Zone. They harmlessly hover around the air, but as soon as the lights go off, lots of them appear, and proceed to attack the player. They can be scared away with a spin, but more ghosts will appear in the process. They won't stop appearing unless the lights are turned on again. The name of this enemy resembles a noise that a ghost would make.