Monsters

Monsters are a class of creature in Undertale. They dwell in the Underground, beneath the Barrier, and Humans live on the surface above the Barrier. Every character encountered in the game besides the protagonist is a monster.

In the Underground
All monsters are trapped within the Underground, ranging from skeletons like Sans, to Spiders like Muffet. All Monsters are ruled by the King, Lord Asgore, as well as previously by their Queen, Toriel.

Monster souls
Monster souls are different from human souls in that they are magical in nature, and compromise the entirety of the Monster's physical being. As such, when they die, they turn to dust and their soul immediately disappears (Boss Monsters excluded).

Additionally, their soul is significantly weaker than a human's, as being physically weaker than a human.