[Eomer of the Rohirrim]

I think JRRT considered Amroth to be the child of Galadriel and Celeborn after he had written LotR. He then thought about it and decided that it didn't fit in with what he had written in LotR. (This being after he had written that bit in The History of Galadriel and Celeborn)

So he thought up some other history for the guy.