Mad Dummy is a ghost monster living inside a dummy located in the Garbage Dump. They also serve as a mini-boss in Waterfall. During a genocide run, the dummy becomes so angry at the player's actions that they become corporeal. This makes the dummy happy but now that they are corporeal, they can be killed.
On a neutral run the dummy gets angry at the player for their interaction with the dummy in the ruins and their interaction with the mad dummy themselves. It doesn't matter what options they select during the fight in the ruins or whether or not the player punches/ignores the mad dummy, the mad dummy will always be mad.