Balkan probably comes from a Turkic word and originally it referred to only a specific mountain range in Bulgaria. Some called it Stara Planina, while others called it Haemus and i don't think that the Balkan term was even the dominant one. A German cartographer later mistakenly assumed it was the dominant range on the peninsula and began using the name more broadly. The reason this region is called the Balkans is due to European cartographers who adopted the term from the German guy. If he had chosen Haemus instead of Balkan, the region might today be called the Haemus Peninsula.