Master Hand

Master Hand appears in only the episode Have you seen this polygon? He appears when after Mario searches

for his missing left hand (polygon) he shoots himself in space using the bomb-omb's cannon. After he flys into space he arrives to see his left hand but it has a mind of it's own so Mario has to defeat it to get it back as a hand.

He also has a really small cameo in a flashback of the hand beating up Luigi and yelling for help.