The Beast was a huge serpent that Sly Cooper encountered in Mz. Ruby's swamp in Haiti. When he explored the Beast's territory in search of a treasure key, he ended up running for his life when he woke up the Beast. Sly later used the Beast to bash through a barricade to gain access to the remainder of Mz. Ruby's production facility.
The Beast is a large and flexible creature with red and yellow scales, as well as red-pink eyes, razor-sharp teeth and a snake-like tongue. Sly's cane would have little to no impact on it. It also breathes a green, toxic-like fume when it roars.
Due to its fearsome and vicious nature, nearly every character feared it, including Mz. Ruby, who kept it contained within a large swamp area, along with one of her treasure keys.