I was struggling a bit with the concept of this creature the other day, ended up contradicting myself a lot because I'd forgotten a lot about the Dark Tower. I'm still pretty sure about some of what I said, but upon organizing my thoughts, I came across a theory that relates to the Dark Tower.
I remember reading somewhere that IT and The Crimson King, while not the same entity, are both essential oppositions to order within the universe. As in, they both bring chaos, while The Turtle, as well as the other beam guardians, bring the opposite.
That would make IT one of the demon elementals, assigned to the Outer Beam. Thus, the Turtle is in alliance with the "White." IT, while still assumed to be a cosmic entity, would be one step below the Crimson King (as far as servitude, at the very least), but still extremely powerful. If IT is more powerful than the Crimson King, I suppose its allegiance would originate from CK's goal involving the Prim, etc. CK has a bit more flexibility.
This would account for Insomnia's
As Robert Gray stated, IT's line of thinking was completely different than CK's - thus, what would remain unexplained within this line of reasoning is the fact that IT was not aware of any other cosmic entity besides itself and the Turtle. I'm not going to venture a guess at why this is (in relation to this theory), since the concept of IT being the beams' antithesis to the Turtle is only speculation to begin with.