marvel - How does Ego know what a celestial is?

Ego says he is a celestial, and tells his story as finding himself alone in the universe, and searching for other life. When he finds that life, he calls it "disappointing", which would seem to indicate it was not life of his species.

But then how does he know what Celestials are? How does he know he is one? He clearly seems to think there are others like him, since he says he is a celestial, not the celestial or anything similar.

For purposes of this question, let's set aside the fact that Ego isn't a celestial in the 616 universe; we're just talking about the cinematic universe. Let's also assume that Ego wasn't purposely lying or otherwise misleading Peter (unless we have strong evidence that he was).

Answer

In the first film, Gamora knows that Knowhere is the severed head of an ancient celestial being.

It's called Knowhere. The severed head of an ancient celestial being.

Knowhere has been inhabited by a lot of people for centuries at least, so this must be common knowledge, and the concept of a "celestial being" must be common knowledge too, or that statement would have no meaning.

The Collector also shows the Guardians an image of a celestial using the stone in their possession to destroy a planet.

These stones, it seems, can only be brandished by beings of extraordinary strength. Observe. [he shows them the giant beings that use the Infinity Stones as weapons] These carriers can use the stone to mow down entire civilizations like wheat in a field.

I think there is also a mural of a celestial behind the Grandmaster's box seats in the trailer to Thor: Ragnarok, though this may just be an easter egg.

So celestials are common knowledge, though we are never told how.

Guardians of the Galaxy 2 spoiler warning.

Ego was originally alone for a very long time, but by the time he meets Peter and tells his story he is very familiar (wink wink, nudge nudge) with life throughout the universe. He has already visited and knocked up women on thousands of planets! If everyone in the galaxy knows about celestials, then the incredibly cosmopolitan Ego is certain to know enough about them to have guessed that he is one too.