King of the Monsters

King of the Monsters is a fighting/wrestling game. It was released by SNK on July 1, 1991 in Japan (later released on the Virtual Console), with later ports for the Super NES and Sega Genesis by Takara. King of the Monsters was included as part of SNK Arcade Classics Vol. 1 which was released for the Wii, PlayStation 2 and PSP in 2008.
Gameplay

Players get to choose any one of six monsters (four in the 16-bit ports) for battle, and two players can join forces to fight the monsters together. Battles end when one of the monsters is pinned for a three count or if time expires (in which case the player loses).

The game consists of 12 total levels: the player first must defeat all six monsters, with the last monster being oneself, but in a different palette. Then the player must defeat the six monsters again, in the same order, but this time in different cities.
The Characters

Geon: A Godzilla-like dinosaur.
Woo: A King Kong-like giant gorilla (not available on the ports).
Poison Ghost: A creature covered in toxic waste, inspired by Hedorah (not available on the ports).
Rocky: A giant Moai-like golem made of boulders.
Beetle Mania: A large beetle-like creature, possibly inspired by Megalon.
Astro Guy: An Ultraman-like superhero with the name reference from Astro Boy.

Battlegrounds

The battles in King of the Monsters take place in six major Japanese cities (one for each monster): Tokyo, Kyoto, Kobe, Osaka, Okayama and Hiroshima. Almost everything in the stage (within the confines of the electrical barriers) can be destroyed. Vehicles such as helicopters, boats, and tanks can be picked up and thrown.
King of the Monsters 2

On May 25, 1992, SNK and Takara released a sequel for the Neo-Geo. Takara also ported it to the Super NES in 1993. This sequel takes the original in a new direction by using three of the monsters from the original game as heroes in a side-scrolling beat 'em up. Geon, Astro Guy, and Woo are all upgraded to defend Earth against an alien threat led by a creature called Famardy. The game also features a 2-player competition mode.
The Heroes

Super Geon: Geon mutated into a spiky, vicious form.
Cyber Woo: A cyborg version of Woo, possibly inspired by Mechanikong.
Atomic Guy: Astro Guy with a super-charged arsenal and bigger muscles.

The Alien Menace

All the monsters had different names in the Japanese and American versions of the game.

Huge Keal (Japanese) or Huge Frogger (American): A giant humanoid toad with a space helmet that can teleport.
Horn du Out (Japanese) or Eifflelyte (American): A blob attached to the head of an ordinary human, who has grown to immense proportions because of it.
Yam a Mordon (Japanese) or Claw Head (American): A mutated creature with two horned heads, arms used as feet, and a large mouth on its belly.
Kili Kili (Japanese) or Beetle Master (American): A large scorpion-like monster with a detachable brain that burrows underground.
Sack Eyes (Japanese) or Aqua Slug (American): A giant mutated sea slug with the ability to inflate and freeze its enemies.
Eat Wow (Japanese) or Lavicus (American): A strange creature with long fingers and a serpentine body that lays eggs from a tube in its forehead. The eye at the center of its mouth has the power to generate suction energy.
King Famardy (Japanese) or Famardy (American): An enormous green, four-eyed blob with lots of teeth and an orange head and arms sticking out of its midsection. Famardy is the final boss and does massive damage by jumping on or devouring the other monsters.

Battlegrounds

The Japanese cities of the first game have been abandoned in favor of more generic areas: American City, French City, Grand Canyon, Desert, Sea Bed, Volcano, and Hideout.
Mega Drive/Genesis version

A Genesis version of King of the Monsters 2 was also released by Takara, but is somewhat different from the other two versions. Rather than being a beat 'em up, the Genesis version plays like a fighting game, similar to its predecessor. All characters (excluding Famardy) are playable and have all been given special attacks executed using standard fighting game motions. Players must destroy all other monsters, one after the other, without any of the scrolling stages in the Neo-Geo version and then defeat a clone of the player's monster.