Big & Little Boo
From the Super Mario Wiki
A Big & Little Boo 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 would mirror the Yoshis' movements, approaching the Yoshi when Yoshi came towards it. It could only be defeated by being tricked into looking away, allowing the Yoshi to throw eggs at the smaller Boos in the same manner as he would to later be able to defeat Big Boos. Each Boo group held a Melon. Big & Little Boos only appeared in Stage 2-3: Jelly Pipe.