From Muppet Wiki

Cosmo Scam is the villain of the TV special The Great Santa Claus Switch. A malovolent magician, he lives in a cave beneath Santa's Workshop, surrounded by toadying creatures known as Frackles. He hatches a naughty scheme to disguise himself as Santa Claus, kidnap the real Santa, and fly around the world burglarizing homes.

When Cosmo Scam wears his Santa costume, he is nearly indistinguishable from Santa himself, due to the fact that both characters in the special are played by Art Carney.