Boo Buddy Swarm first appears in Yoshi's Story, so far being their only Yoshi-related appearance.
In the game, these foes appear in the Ghost Castle. The Boo Buddy Swarm is made up of tiny Boos. They move along past, sometimes hitting Yoshi, but if Yoshi hits one of the group members with an egg, they all come swarming at Yoshi. When the Boos move off-screen, more join the group.