@texczech82 Serbs and Croats were the names of two Slavic tribes that settled and expanded in the Western Balkans. Bulgarian, well that’s named after the Bulgars, which were either a Turkic or Iranian tribe that became the military elite of a new State/Empire in Eastern Balkans, of which, occurred around the time of the Great Slavic migration into the Balkans, hence the Bulgars and Slavs teamed together in this instance, but this mixed race became slavicised over time, no-one really knows how large the genetic make-up was attributed by Bulgars, it could be negligible.
Macedonian is well, named after an earlier state, during the Hellenistic period in the region, prior to the Roman Empire conquests. Macedonia has been a regional name for its area under various regimes through the ages.
Montenegro, Bosnia and Slovenia are others too, but I’ve not read so much on their history, just the basics. Montenegro means Black Mountain in Latin, so it also goes by the name Crna Gora as well.
All these slavic states mixed with the indigenous populations form the 5th Century onwards in the Balkans. The Balkans had largely been depopulated from various incursions, and apparently the population of the entire Balkans was estimated at 3.5m people back then; making it quite easy for the Slavs to settle and soon assert their cultural dominance in the region, by the 15th Century, the population was closer to 8.5m people in the Balkans.