In Final Fantasy IX Kuja's most powerful summon was Invincible which he usedto enslave Bahamut. however during the attack on Alexadria when Kuja tries to use it to enslave Alexander, Garland shows up taking back control and revealing that the Eye that appeared in the sky was actually an airship.
at first i thought that it was because Garland took control but when Madain Sari was destroyed The Invincible took the form of an eye here and this time Garland wanted to destroy the summoners, fearing their Eidolon.
But why does it take the form of an eye? if it was just to hide the fact it's a machine it's not like an Eidolon can't be a machine as Ark and Alexander are machines and Eidolons