The Monster Rancher video game and anime franchise features monsters raised by monster breeders, who then fight in tournaments against other monster breeders.
Each of these monsters are uniquely defined by a primary breed (such as Golem), which defines its basic form and abilities, and a sub-breed, which defines its color scheme, modifies appearance, and slightly alters some of its statistical capabilities. Breed is commonly specified as Primary/Sub because of the inconsistency of name and form for the generated monsters between the games. For example, Golem/Suezo indicates a monster with primary breed Golem, sub-breed Suezo, and manifests as a large humanoid figure with the bright yellow skin and single eye that are characteristic of the Suezo monster breed. Each monster breed also has at least one "rare" sub-breed. These often unusual sub-breeds cannot be created by combining. They only come from certain CDs. For example, in monster rancher 2, inserting the monster rancher 1 disc creates a rare sub-breed of Suezo called "Seuki-Suezo", which is Seuzo/???. All Rare sub-breeds are their main breed/???. Below is a list of many of the different species used in the franchise. Not all appear in every game, and many are culled for the sequels.
According to the story presented at the start of Monster Rancher 2, the monsters were created by God in response to a series of natural disasters humans found themselves incapable of dealing with properly. Once monster use became commonplace, the humans turned them into weapons of war. To limit these problems, God sealed the monsters into the disc-shaped stones called 'saucer stones'. An untold amount of time later, the Tochikan tribe (introduced in Monster Rancher 3) discovered the method for unsealing the monsters. However, due to their xenophobic nature, they refused to share this technology until after the events of Monster Rancher 3 had ended.
Within the storyline of the anime, monsters were created through genetic engineering as designer pets, until a war broke out in which the monsters were modified into biological weapons which could have their physical forms stored on discs. Although the ensuing conflict destroyed much of the world's civilization and technology, later generations used the discs to recreate the monsters to be used as pets, beasts of burden, and created a sport in which they were used as competitive fighters.