Wacko is a 1983 arcade game by Bally Midway. It debuted during the "Golden Age of Arcade Games". It featured a unique angled, or "sloped" cabinet design and a combination of trackball and joystick controls.
Shooting a different monster, on later boards, results in the two becoming a mutant -- the head on one joins with the torso on the other, and vice versa. To eliminate these mutants one must either "unmix" them, by shooting the same pair again, or produce a second set of identical mutants which can then be paired up with the respective members of the first pair. Eliminating a pair of mutants is worth more points than eliminating a pair of non-mutants.
At more advanced levels a third stage is introduced where the monsters become tiny and sprout wings.