@feha Hmm, actually, Egbert is a real name, an old Anglo-Saxon name. There has been several rulers in England with that name. Ethelbald is also an old Anglo-Saxon name and was the name of a few monarchs in England. As for Owen, it's an old English name that was used by the nobility (mostly in Whales from the 15th century onward), its etymology traces itself back to Eugene, meaning "noble-born."
For the "fertile" thing, it refers to the kingdom. It's an old tradition in most monarchies for the king's surname to be the name of the kingdom, or sometimes for the kingdom's name to come from a new ruler's surname.
Finally, the "Rex" in the middle of the name is just the latin word for "king." I assume the author added it there to make his name more fancy.
@Thiron No, Gus said that it was an old Elven tradition. Menel doesn't have that likely because his entourage didn't like him and they didn't know his father's name. Maybe he has a his mother's name as his surname and simply hasn't told anyone what it is, but he might also just not have an official surname based on his upbringing