The Demon type is one of the many fanon types from the Pokémon franchise. They are generally an offensive type with not many specialties in other types, although many of them have a good Speed stat. It is the rarest type, with only 17 species being part of it (14 if Mega Evolutions are not counted). It was primarily made to balance out the Fairy-type.
A Demon-type Pokemon, on average, have very high Attack stats, decent-good Speed and Special Attack stats, and bad Defenses.