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 three Technical Points (TP), 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 and obtain. The Shiva Sisters, after being defeated, can be used by Snow and only Snow. Each Eidolon also has a Gestalt form. When Gestalt mode is activated, 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 are Snow's Eidolon. When in Gestalt Mode, they merge together to make a motorcycle. Matching Snow's first 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.

Odin
Odin is Lightning's Eidolon. When in Gestalt mode, he turns into a horse. His moves are charged with lightning.

Brynhildr
Brynhildr is Sazh's Eidolon. When in Gestalt mode, she turns into an armored car. Her attacks are dealt with guns.

Bahamut
Bahamut is Fang's Eidolon. When in Gestalt mode, he turns into a dragon. His moves are based on his fiery wrath.

Alexander
Alexander is Hope's Eidolon. When in Gestalt mode, he turns into a fortress with cannons. He uses his fists to knock out opponents.

Hecatoncheir
Hecatoncheir is Vanille's Eidolon. When in Gestalt mode, he turns into a dual-sided large-barreled gun. He uses his harpoon-like arms to face his foes.