In Ghostbusters and other films of the time, a combination of animatronics, traditional puppetry, sculpture and design were used to create 'creatures'.

The 'creatures' and ghosts in Ghostbusters would be produced in many different versions: a static sculpture, scaled-down version, animatronic version, and some like Slimer and the Library Ghost needed to be worn by a performer.

“His mouth and eyes were operated by puppeteers dressed in black, sitting below and behind him. Someone else’s arm operated the tongue. The camera was locked off and we would move ‘Slimer’ around according to the shot." VFX supervisor John Bruno