They don´t ride dragons
anymore iirc.
The main problem with the otherwise sensible idea of The Empire of Man is that they have Emperor, which has only been true for like a third of its existence, as they have somehow had two to three at a time for the other two.
. It would also make ruler of Elves the Phoenix Emperor, not Phoenix King. Turns out names can be a bit stupid, huh?
But anyway, even if they have a king, prince can still be just a noble title thats two rungs lower than a king. There being kings changes nothing with the explanation provided by Rex.