Each Monster species is 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-type 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 can only be unlocked directly at the Shrine. For example, in Monster Rancher 2, inserting the Monster Rancher 1 disc creates a rare sub-breed of Suezo called "Sueki Suezo", which is Suezo/???. All Rare sub-breeds are notated as Primary/???. Hibernating monsters can be combined. If breeders combine them, their different uniques skills will be used, and it can be really easy to produce a strong monster.
In almost every game, new monsters can also be created by combining two of your present monsters together to produce a single monster with features from both parents, including physical attributes, traits, and stats.
However, in Monster Rancher 3, this feature was replaced with a system of "evolution" which altered the monsters according to the type of area they train in. In Monster Farm Online and Monster Farm Lagoon, items and the type of food you feed your monster can alter their traits and appearance.
In the anime series, combining allows two monsters to fuse together with the main monster's soul taking over the new body while the sub monster's soul ceases to exist. In Episode 47, Big Blue asks the Searchers to fuse him with Pixie in order to save her life after she is badly hurt. The combination succeeds and Pixie is transformed into Granity. Their outward traits meld together, but just as importantly Big Blue's spirit remains within her and saves her from Metalner's possession.
Master Moo's possession of Holly's father and later General Durahan and his lackeys is similar to the combining process as it also creates a new monster. However, both of these instances are shown to be reversible.