Mad Monster Party is a 1968 Halloween-themed children’s film created by the Rankin/Bass animation house and written by Mad magazine’s creator, Harvey Kurtzman (with Len Korobin).

Rankin/Bass were famous for their stop motion Christmas favorites like Rudolph the Red-Nosed Reindeer and Frosty the Snowman. The film was directed by Jack Bass in their signature “Animagic” process.
 

 
Many of the characters in Mad Monster Party were designed by Mad’s Jack Davis, a man well-suited for the gig by his earlier comedy/horror work in the pages of EC Comics. The film featured the vocal talents of Boris Karloff and Phyllis Diller who were represented onscreen by their own likenesses.

Mad Monster Party was very influential on Tim Burton’s short film Vincent, The Nightmare Before Christmas and Corpse Bride. Some of the monster characters in Corpse Bride seem to be in homage to the earlier film. It would also appear that the Sesame Street character, “The Count” made his first appearance here, too…