The Halloween That Almost Wasn't (1979)
By the late '70s and early '80s, no one was taking the classic monsters seriously anymore. They'd been replaced by demons and serial killers and banished to children's programming and comedy bits. The Halloween That Almost Wasn't represents the former group, although according to the indispensable Frankensteinia, it's a very entertaining example.
Judd Hirsch plays Dracula and rounds up a bunch of other spooks - including Universal's version of the Frankenstein Monster - to convince Mariette Hartley's witch character to fly across the moon and kick off Halloween. There's slapstick, adorable moppets (not muppets; be patient), and disco. John Schuck, who played the Monster, went on to play bit parts in various Star Trek TV shows and movies (he's probably best known as the Klingon ambassador in Star Trek IV and VI) as well as Herman Munster in the reboot series, The Munsters Today.
The Muppets Go to the Movies (1981)
As part of the promotion for The Great Muppet Caper, ABC aired a one-hour TV special called The Muppets Go to the Movies in which Lily Tomlin and Dudley Moore helped the Muppets honor classic films like The Three Musketeers, The Wizard of Oz, Casablanca, Tarzan movies, and of course Frankenstein. The "Nephew of Frankenstein" bit featured Fozzie as the title character who interrupts his uncle (Dr. Strangepork from "Pigs in Space") in the creation of his Monster (played by Mulch, who's probably most famous for playing the Hunchback Notre Dame and chasing Debbie Harry around as she sings "One Way or Another" on her episode of The Muppet Show.)
It wasn't the only time the Muppets and Frankenstein hooked up. Miss Piggy and Kermit reenacted a scene from Bride of Frankenstein for the 1982 Miss Piggy calendar, the Monster's shown up in a couple of episodes of Muppet Babies, and IMDB mentions an appearance in next month's The Muppets.