Eidolon

Eidolons are magical creatures called into battle by a Summoner. Eidolons are the name of Summons in in several Final Fantasy games: the Nintendo DS version of Final Fantasy IV, Final Fantasy IX, and Final Fantasy XIII. The Summoners of Mist in Final Fantasy IV, the Summoners of Madain Sari in Final Fantasy IX, and the l'Cie from Final Fantasy XIII are the only people who can Summon Eidolons.

Final Fantasy IV
Originally known as simply Summons, Eidolons are a race of intelligent monsters that live in their seperate world known as the Feymarch. Rydia is the only party member who can summon Eidolons.

Final Fantasy IX
Eidolons are massively powerful creatures in Final Fantasy IX, with enough power to destroy whole cities. For that reason, they are targeted by people greedy for power. The power of Madain Sari are the only ones who can Summon Eidolons, unless their power is extracted. They play major roles in the game's plot. Dagger and Eiko are the only party members who can use Eidolons.

Final Fantasy XIII
Eidolons in Final Fantasy XIII are magical beings that can be summoned by the playable characters who are given the powers of the l'Cie. Each Eidolon is unique to their summoner, because they reflect the summoner's personality.

They have the power to transform into mechanical devices via a system called Gestalt Mode. Shiva consists of two sisters who are able to morph into a motorcycle, while Odin can transform from a knight into a horse. Brynhildr, a new female fire-elemental knight, transforms into a race car. Bahamut takes flight in his Gestalt Mode.

Carbuncle has also been shown emerging from a disc. Other Eidolons revealed include Ifrit and Siren.

Etymology
Eidolon is the Greek word for "image" or "specter", from which the Latin word Idolum comes. And Idol is "a representation or symbol of an object of worship."

Eidolon