Warlock recruits the most vicious and powerful monsters into his army of Evil Monsters in order to defeat Vampire and his Good Monsters.  They trick the Good Monsters into attending a meeting on Monster Mountain, where they shrink the heroes.  Unfortunately for them, they are also shrunken.  They proceed to cause chaos for the Good Monsters as much as they can.   The Evil Monsters consist of Warlock, Medusa, Spring-Heeled Jack, Cyclops, Ogre, Tengu, Karnak, Ghost, Windigo, Siren, Charon, and Spectre.  It is unknown if the other participants of the Convention of Evil -- Great Beast, Ymir, Gremlin, Kali, Manticore, Minotaur, Witch, Triton, Kraken, Catoblepas, Griffin, Cockatrice, Skeleton, Zombie, Behemoth, Haniver, Red Cap, Hunchback, Centaur, and Ghoul -- were members of the Evil Monsters, or if they remained neutral after being shrunken.
 

Other Media:

• In the animated special "The Big Scream," the Evil Monsters are Vampire, Medusa, and Swamp Beast.  Vampire and Swamp Beast, in the Harvey comics, were members of the Good Monsters, leaving only Medusa as the only member of the Evil Monsters in the special.
• The Evil Monsters appearing in "Monster in My Pocket: The Video Game," are many of the same from the comics.  There are additions to the villains, such as Baba Yaga, Hydra, Bigfoot, and Harpy.