Lothar is a dragon-like monster who works for Cosmo Scam in the 1970 special The Great Santa Claus Switch. Lothar apparently lives in the cauldron of Cosmo Scam, and can fly.
Lothar later made an appearance on The Dick Cavett Show, and also appeared in the audience of the wrestling match in The Muppet Show: Sex and Violence and in The Muppet Show episode 217 (Cold Open) and 324 ("There's a New Sound") where he was refurbished with different eyes, fuzzier purple skin and a brown wig.
In The Muppets Take Manhattan, Lothar made a cameo at the wedding of Kermit the Frog and Miss Piggy.
Lothar from Episode 217: Julie Andrews.
Lothar from Episode 324: Cheryl Ladd.