Final Fantasy XIII/Eidolons

Not only are there boss battles in the game, but also Eidolon battles. Eidolons are your summons--if you have at least 3 Technical Points, you can summon your Eidolon to help you out in battle. An example is Snow's Eidolon, the Shiva Sisters, who are also the first Eidolons you fight. The Shiva Sisters, after being defeated, can be used by Snow and only Snow. Each Eidolon also has a Gestalt form. Odin, Lightning's Eidolon, turns into a horse when Gestalt mode is activated, while the Shiva Sisters form to make a motorcycle. Eidolons can really help you out in battle, whether they're fighting with you and standing by your side or you're riding them and unleashing hell on your enemies.

Shiva Sisters
The Shiva Sisters is Snow's Eidolon. When in Gestalt mode, they form together to make a motorcycle. Matching Snow's name, all of their their moves seem to involve ice. Below is a table of moves the Shiva Sisters can perform while in Gestalt mode.