Uncle Deadly is introduced in this show, despite having already appeared in Vincent Price (1977), which was recorded earlier. The Muppet Show originally aired in syndication, and aired in a different order in each market, so the audience in some markets may have seen this "introduction" for Uncle Deadly after seeing him appear with Vincent Price.
King Rupert is a classic Muppet that had evolved from a character named King Goshposh. Goshposh had appeared in Tales of the Tinkerdee (1962) and Hey Cinderella! (1969), and was given a makeover and name change when Tales from Muppetland: The Frog Prince (1971) was taped.
While most of the Muppet monsters in the show's first season had evolved from pre-Muppet Show trunk puppets and Frackles from The Great Santa Claus Switch (1970), Uncle Deadley was created for The Muppet Show and his design is more elaborate and frightening.
Miss Piggy was not in this episode.