The Master Monsters are a collective group of mysterious creatures that were unleashed in the portal by Master Hand. They are secondary bosses in SSB4
The 5 monsters that appear in some prominence in the game include:
Bigfoot: The large, blue-furred ape-like beast that the Fang Ninjas encounter as Shulk attempts to capture it for Meta Knight.
Cerberus: The grey-furred, fire-powered canine-like creature that the Fang Ninjas encounter in a great hall.
Goliath Frog: The dark green frog-like creature that appears in Congo Jungle.
Jersey Devil: The red dragon-like creature that Pit encounters in an old house.
Chupacabra: The huge, black spiky-shelled reptilian monster that the Blade Generals encounter on an airplane.