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 normal Big Boo, followed by Boo Buddies. Big & Little Boos would mirror the Baby Yoshis' movements, approaching Yoshi when Yoshi came towards it. It could only be defeated by being tricked into looking away, allowing 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 Buddy group held a Melon. Big & Little Boos only appeared in Stage 2-3: Jelly Pipe.