So, like many, my first introduction to the term "Celestial" was Knowhere in Guardians of the Galaxy, which is said to be the head of a Celestial. Then I'm pretty sure they state that Ego is one in Guardians of the Galaxy Vol. 2. Since Ego was one of a kind, I took it to mean it as something of a blanket term to cover great cosmic beings like The Watcher, etc.
But, in order to prepare for The Eternals movie, I started reading The Eternals comic book, and the Celestials therein are definitely their own species, of which Ego is not a member.
I decided to look up if Knowhere existed in the comics. It does, but it has a weird section on the wiki page on how it's first appearance was "historically" in 2008, and "chronologically" in 2018, well after it was intoduced in the movie. So what does this mean? Was it invented in the movie and retroactively stated to have been in the Marvel Comics Universe?
At any rate, it seems that Knowhere was always meant to be the head of an Eternals-kind-of-Celestial, and Ego was not.