Aliens that are "near Human" but have some kind of growth or funny forehead shape.

Named after the common practice of Marvin Westmore to make aliens with upper facial appliances in order to allow the actors to retain their "working tools".

It became a running joke but was rather a necessity. When Gene Roddenberry sent to Central Casting for non-human actors, they sent Lassie, Flipper and Mister Ed.

In short, TV and film had to work with the rather limited supply of Humans wearing funny colors and bits of rubber in the days before CGI was dirt cheap, or even existed.