If I have eye of Ugin on the field or a Animar with 2 counters on him, can I put down a {Endless One} with two counters on him?
Yes to both. When you cast an X spell, you declare what x is before costs are paid. Any cost-reducing effects then apply. If you have turn 1 {Eye of Ugin}, you can play {Endless One} for 2.
Man that's gonna be sweet!