Mad Dummy

 is a ghost monster that lives inside a dummy located in the Garbage Dump.

Appearance
Mad Dummy takes 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. However, as Glad Dummy, they will instead have two happily closed eyes and a smile.

Neutral Route
When is encountered at the Garbage Dump in Waterfall, they reveal that the dummy in The Ruins was possessed by their cousin.

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 the protagonist's soul. The fight is later stopped by Napstablook. After the fight, they will be found outside of Undyne's House taking a job as a punching bag.

Genocide Route
When encountering in the Genocide Route, the amount of hatred directed towards the protagonist's actions allows them to "fuse with their body", fulfilling their lifelong wish. As a reward, they're willing to spare the protagonist as soon as combat begins. Unfortunately for the newly-renamed Glad Dummy, they're now vulnerable to physical attacks.

If the protagonist decides to abandon the route for the Glad Dummy, they can be found outside Undyne's House and will continue to thank the protagonist for their corporeal form.

In Battle

 * See Mad Dummy/In Battle

True Pacifist Ending Credits

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

Trivia

 * makes several possible references to JoJo's Bizarre Adventure: Stardust Crusaders.
 * They yell out "Foolish. Foolish! FOOLISH!", "Futile. Futile! FUTILE!", and "Pitiful. Pitiful! PITIFUL!" Similar to how DIO would repeatedly yell out . They also pull out and throw a knife which alludes to how DIO pulls out and throws a multitude of knives.
 * In addition to this, 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 protagonist's soul green.
 * It's stated that one of Napstablook's cousins also went to possess a training dummy, but it's not confirmed if either or their cousin is related to Napstablook's cousin at all. In fact, after interrupting the battle, Napstablook refers to  as the protagonist's friend, suggesting that the two might not be related.
 * Like the Dummy in the Ruins, repeatedly checking Glad Dummy will make them tired of the protagonist's aimless shenanigans and leave.
 * At the end of the fight, tries to defeat the protagonist by holding their turn forever. This is later used by Sans at the end of a Genocide Route, but both times the technique fails.
 * The beginning of the fight is a reference to the Guts Man scene in the Mega Man cartoon.
 * Most in-battle flavor text refers to Mad Dummy as an "it", but Mad Dummy's check data calls them a "they".