Relay Boo Guy is an enemy in the Yoshi (series) debuting in Super Mario World 2: Yoshi's Island. They're bomb dropping Boo Guys.
Super Mario World 2: Yoshi's Island[]
Relay Boo Guys can be found stationed in long rows within ceilings of some castles as their name implies they relay a purple bomb which they attack by dropping it on Yoshi when he approaches them ground pounding near them will force them to drop the bomb due to their position they cannot be defeated.
Yoshi's New Island[]
Relay Boo Guys appear in Yoshi's New Island. They act the same from their previous appearance, once again attempting to dropping their bombs on Yoshi.