A companion beast to the Ultima Weapon. It appears as an optional boss that is usually very difficult to beat without a high level of experience. Its appearance teeters between a four-legged robotic machine and a palette swap of Ultima Weapon (who is vaguely centaurian). Often the only reward from defeating him is a special/key item - "Proof of Omega" - which has no use except bragging rights. In FF10, Omega managed to get an entire dungeon named after him - the Omega Ruins - but is nowhere near as hard as his previous versions. In FFX-2, his reputation is demeaned by becoming a randomly encountered monster.
-Omega (Dawn of Souls)
-Omega Mk. II (GBA only)
This is where Omega first showed up, as an amazingly tough boss in the Interdimensional Rift. In the GBA version, there's an even tougher brother of Omega in the Sealed Temple - and to get to him, you'll have to wade through no less than FIVE of the original Omegas!
-Omega Mk. XII
One of the three hardest fights in the game, this monster can only be fought in the Great Crystal of Giruvegan after gaining all Espers and accepting the Yiazmat hunt. Unlike the previous versions, this one has a backstory revealed in the Clan Primer. Originally created by a scientist to fight a great wyrm (unnamed; perhaps Yiazmat?), and given the ability to grow independently via an invention called the Growth Lattice, the scientist left the Omega Mark XII alone, having no idea just how much it would grow.
CP: An ancient weapon of war, traveling the rifts between dimensions. Its body is set with machineries and weaponry of a far higher order than anything made today. Though it is a construct, it was built with the ability to grow, and so it has evolved like a living creature. It generates the energy it needs to live by filtering and condensing Mist from the air.
Yes, I know "omega" and "upsilon" are two different Greek letters, but it's quite obvious this one is meant to be a continuation of Omega Mk. XII from FF12, especially if you read the rumors.