After WW I Yugoslavia was created ((Land of the South Slavs), an amalgamation of Serbia, Croatia, Slovenia, Bosnia/Herzegovina, Montenegro & Macedonia. It officially lasted until 2003 (Though in 2003 it was only Serbia & Montenegro.) Serb dominated throughout its existence, when it was a kingdom it used the Serbian royalty. I think its last King was King Peter.
Thanks, I knew someone here would know the history. It makes sense now, Yugoslavia was formed after WW1 and the existing monarchy of Serbia used for the entire country. I bet the Croats weren’t happy with that.