Evil-Mix (or, as identified in the game, Monster) is the twisted result of an accidental fusion between Bowser, Dry Bowser and other 4 bosses. It's the extra boss fight in Wario Land 4 when after beat bowser second time. It was created when the Planetary Minimizer malfunctioned (courtesy of Crash) and fired a laser at the villains, causing them to combine and mutate into the twisted shape of the Evil-Mix.
The Mega-Mix is a combination of the head of Cractus, the peak of the head of Cuckoo Condor, the torso and arms of Dry Bowser, the shell of Bowser. the legs and feet of Aerodent and the tail of Spoiled Rotten. After the transformation, Evil-Mix chased Wario through the hazardous space station, until Wario made it to the exit and escaped with a space motorcycle. Evil-Mix was "destroyed", when the space station exploded, separating the villians' bodies from each other.