Anything Muppets (sometimes referred to as "AMs") are "empty" Muppet heads. Facial features, bodies and clothes can be added to make any kind of character. The corresponding family of monster puppets are known as the AM Monsters.
A similar group of puppets known as Whatnots appeared on The Muppet Show. Some Anything Muppets have also appeared in Muppet movies and specials.
The Anything Muppets were introduced in the first episode of Sesame Street, in a sketch involving Gordon. An Anything Muppet appeared without any facial features, and Gordon introduced her as an Anything Person and proceeded to give her and four other Anything Muppets who appeared in sequence eyes, noses, and hair to form a family.
During the first seasons of Sesame Street, they were known as "The Anything People" who could be anything they wanted. Usually the live actors, like Bob, would turn them into different characters. This concept was abandoned after a few seasons, although every once in a while an Anything Muppet character would take off his nose or lose his mustache.
“Our general philosophy on "Sesame" is that we've always dealt with not being specific to races. That is why Jim Henson developed artificial colors or strange colors for all the humanoids.” —Ed Christie describing Anything Muppets to the crew of Sisimpur[1]
Fat Blue Anything Muppet from "The Muppet Character Guide" (1973)
Lavender Anything Muppet from "The Muppet Character Guide" (1973)
The term "AM" is also used to refer to various background characters, such as monsters, animals, Honkers, Dingers, and other creatures. In these cases, scripts call AMs "animated Muppets."