Bowser Memory Boo
From the Super Mario Wiki
Bowser Memory Boos are enemies that appear in Mario & Luigi: Bowser's Inside Story during the battle with Bowser Memory ML. They appear as one of Bowser Memory L's attacks they look like generic Boos but are square shaped and pixelated. During the attack, Bowser Memory L gets chased around the room and will always hop over the bros while the Bowser Memory Boo homes in on him. Mario and Luigi have to repeatedly jump over the Boo and the attack will end after a set amount of jumps or until a bro stomps on it. After the attack ends, the Bowser Memory Boo flies away. Bowser Memory Boos reappear again in the battle with Bowser Memory MLX in The Gauntlet. They are faster this time and do more damage than last time.