Replikon ask Xum was an adaptable alien with a gaseous form and the ability to shape change. Transforming into a blended form inspired by Batman, Flash and Wonder Woman, Xum sought out the means to transform Earth-One into a world in which his eggs could hatch by terraforming it into his long extinct native world.
Egon was an alien himself hatched from an egg when it landed on Earth-Two and raised by the vengeful Noel Racthford, who sent it once it grew into a giant to frame Noel's rival, Carter Hall... aka the Hawkman. Egon gained a humongous body and powerful eyes unleashing intense bolts of energy towards any matter he gazes upon.
Replikon first battled Green Arrow and Black Canary, then their ally Green Lantern who defeated him and vanquished him to the astroid belt between Jupiter-One and Mars-One. Meanwhile, Egon who had originated from Neptune-Two was defeated and died at the hands of Hawkman after the hero's fiancee Hawkgirl became involved in Noel's scheme.
Eventually, Replikon's two eggs hatched when two other Green Lanterns helped transform his astroid into a habitable home form for all three. If there were any Neptunians besides Egon and his long lost parents, they have yet to surface in any chronicled tales henceforth.