Big & Little Boo
From the Super Mario Wiki
A Big & Little Boo, also called Boo Clusters, is a variant of a Big Boo that appears in the sewer levels in Yoshi's Story. They are named for the fact that there is one Big Boo, followed by normal-sized Boos. Big & Little Boos mirror the Yoshis' movements, approaching it when Yoshi came towards it. It can only be defeated by being tricked into looking away, allowing the Yoshi to throw eggs at the smaller Boos in the same manner. Each Boo group holds a Melon. Big & Little Boos only appear in Stage 2-3: Jelly Pipe.