Who were the previous "Emperors of the Sea"
I don't think previous Yonko have ever been mentioned in One Piece. It would actually be quite odd if there were always exactly 4 strong pirate crews. Especially since they have already added Luffy to the bunch so it's technically not even Yonko anymore.

The title probably originated after Roger died and a few of the pirate crews dominated everyone else. But there isn't much to cite from the manga as Oda hasn't really revealed anything about Shanks' rise to the top.