Ego was a bit strange as a Celestial - he wasn’t one in the comics and how he is depicted in the MCU is inconsistent with the other Celestials in both formation and appearance.
Perhaps they were originally trying to say that Ego was a “celestial lifeform,” rather than a literal Celestial, but that got muddled at some point.
Perhaps at some point he came across Knowhere, learned that it was a dead Celestial and concluded that he must also be a Celestial as it was the only thing he’d come across on a similar scale to himself.
