If I correctly remembered, Ego is one of the on-and-off allies of the Elders because the Living Planet might be one of the oldest living beings in this version of the universe. That's why Ego once helped the Elders in a plot involving the Silver Surfer and the Infinity Stones (when they were still known as the Soul Gems).

The MCU makes Ego one of the relatives of the Celestials. Still, this idea also comes from the comics.

Apparently, Ego the Living Planet came from the Black Galaxy, where a new Celestial was also born
http://marvel.wikia.com/wiki/Black_Galaxy