Mad Dummy

Mad Dummy is a ghost monster that lives in a dummy in the Garbage Dump.

Appearance
They take the appearance of a dummy with a set of upper teeth on the torso, a set of eyes on the head, and nostrils on the nose.

Main Story
When the Mad Dummy is encountered at the Garbage Dump in Waterfall, they reveal that dummy in The Ruins was possessed by their cousin.

Pacifist/Neutral Route
Regardless of how the protagonist interacted with the first dummy, Mad Dummy will be enraged by it. They attack the protagonist with the intentions of stealing their soul. The fight is later stopped by Napstablook.

Genocide Route
When encountering the Mad Dummy in the Genocide Route, the amount of hatred directed towards the player's actions allows them to 'fuse with their body', fulfilling their lifelong wish. As a reward, they're willing to spare you as soon as combat begins.

Unfortunately for the newly-renamed Glad Dummy, they're now vulnerable to physical attacks, and a dedicated player will kill it in one shot.

In Battle

 * See Mad Dummy/Encounters

True Pacifist Ending Credits

 * White Text - "Changed Name to Angry Dummy"
 * Yellow Text - " Calmed Down... Just a Bit "

Trivia

 * Mad Dummy makes several possible references to JoJo's Bizarre Adventure: Stardust Crusaders.
 * He yells out "Foolish. Foolish! FOOLISH!", "Futile. Futile! FUTILE!", and "Pitiful. Pitiful! PITIFUL!" similar to how DIO would repeatedly yell out "MUDA! MUDA! MUDA!". He also pulls out and throws a knife which has been alluded to how DIO pulls out and throws a multitude of knives.
 * In addition to this, the Mad Dummy is later seen outside of Undyne's house, taking a job as a punching bag. This arrangement may be another reference to DIO and his Stand, The World, as both are adorned with green hearts while Undyne can turn the player's soul green.
 * Dummy!, the music played during Mad Dummy's fight, is a remix of Ghost Fight.
 * It's stated that one of Napstablook's cousins also went to possess a training dummy, but it's not confirmed if either Mad Dummy or his cousin are related to them at all.
 * If you continue to check the Glad Dummy, they will tire of your aimless shenanigans like the regular dummy and leave.