Mash

Mash is a giant, round robot summoned by Dr. Wily who grabbed Guts Man in the Robot Museum

Mega Man 7
Mash will hop around the room trying to kill Mega Man by squashing him. Shooting the head makes his head flies and some shots damage Mega Man. His head will land on him by hopping to get the head back. He is immune to all of the weapons from the Robot Masters and can only be hit by the Mega Buster.

Trivia
Mash and Clown Man are the only clown robot bosses in the Mega Man series.

This isn't counting the Pierobot as it's not a boss.

When battling Mash, you can see Skull Man and Ring Man if you look closely in the background.