Eidolon (Final Fantasy IV)

Eidolons, known previous to the Nintendo DS remake as Summons or simply Monsters, are the Summoned Monsters of Final Fantasy IV. Eidolons are clearly differentiated from other monsters. Eidolons are intelligent, can speak English, and refer to the monsters Cecil and his companions fight as their cousins. They build shops, manufacture and sell items to the rare traveler that finds their world, write books, and so forth. Because of their isolation, Eidolons are naturally wary of humans besides Summoners. The bond between an Eidolon and their Summoner is unique. The Summoner must first defeat an Eidolon in battle, then they can use them as a Summon. If the Summoner's Eidolon is killed, the Summoner dies as well.

The Eidolons live in the Feymarch normally inaccessible to humans, although a portal to this world exists in the Underworld and Leviathan is able to travel to and from the surface world by an undisclosed means. In this world time apparently flows faster than normal, and a few weeks or months in the human world can be years in the Feymarch. According to the books in the library in this world, Leviathan and his queen Asura are the rulers of the world, but Bahamut is their God and the ruler of all monsters. Bahamut lives on the Red Moon.

Young Eidolons are called Whytkins, and take the form of Rydia's Eidolon Whyt. In this form, they can grow to take on the forms seen in the other Eidolons. Whyt and his parents stay Whytkins so that new Eidolon children can be born. Though Whytkin have no abilities of their own, they have hidden potential that allows their power to be greatly increased. Not all Eidolons grow up from Whytkin, however--at least one, Odin, is born from a deceased Dark Knight instead, through undisclosed means.