Svengali is a sorcerer-type Whatnot muppet, dressed completely in black with a black moustache, black hair and black eyebrows.
He appeared in the Joel Grey episode of The Muppet Show during the "Comedy Tonight" number. He sang one line, and was performed by John Lovelady.
He later appeared in the Peter Ustinov episode, where he kept turning a monster-like character into various things as she sang "You Do Something to Me."
He was named after a fictional hypnotist from George du Maurier's 1894 novel, Trilby, which was the foundation of today's stereotype of hypnosis.