Marvel Comics started producing calendars in 1975. For the very first calendar (which I've been able to acquire recently, thanks to eBay), they tapped the best artists of that year...including Mike Ploog, who they unleashed in the month of October.
The October illustration featured the trio of monsters that Ploog had drawn in their own comics: Frankenstein, Man-Thing, and Werewolf by Night! The big bad daddy of all supernatural characters, Dracula, lorded over all of them--Ploog had drawn this character as well, when he teamed up with Jack Russell in Werewolf by Night #15.
The month of October itself is ridden with monster-sized jokes by Marvel staffers.
Oct. 4th: Man-Thing with the caption: Save the Everglades--Or Else! That scares me into taking care of the environment.
Oct. 8th-9th: Son of Satan asks pop for the hell-horse drawn chariot, but sister Santana asked for it first.
Oct. 13-15: Three nights of the full moon with Werewolf by Night. Jack Russell's eyes go left, middle, right.
Oct. 19th: Jim Starlin's birthday. Which is today, Happy Birthday Jim!
Oct. 28th-29th: Two creatures called IT! The first one was featured in Supernatural Thrillers #1, the second one was a hero who appeared briefly in Astonishing Tales.
Oct. 30th: The Halloween party in Rutland Vermont, which was featured by Roy Thomas in both Avengers and Defenders (Marvel Feature #2). Several comics creators made the trek to this party. If only we could go back in a time machine!