Eidolon (Final Fantasy XIII)

Gestalt Mode is a special ability that the Eidolons of Final Fantasy XIII possess. It allows them to become a vehicle like from that their summoner can ride

Odin
Lightning's Eidolon, Odin, can become a horse.

Shiva
Snow's Eidolons, the Shiva sisters, can become a motorcycle.

Brynhildr
Sazh's Eidolon, Brynhildr, can become a sports car with side mounted machine guns.

Bahamut
Fang's Eidolon, Bahamut, can become a flying form.

Summons with Unknown Gestalt Modes
The following summons have been confirmed to appear in Final Fantasy XIII, but their summoners and Gestalt Modes are still unknown.
 * Carbuncle
 * Siren
 * Ifrit