The Mutant is one of the more common enemies in Dark Castle, they appear in a number the levels, and show up in Dark Castle, Beyond Dark Castle, Color Dark Castle, Dark Castle Mobile, and Return to Dark Castle.
If you get touched by this enemy, you die, in addition, in the Helecopter Pack levels, they throw rocks at you.
The sound the Mutants make, "Nya Nya Nya" is considered one of the most memorable sounds of Dark Castle.
For killing a Mutant you get 50 Points.
The Mutant have stayed looking the same since the original Dark Castle, with the only real changes being color.