Was there a real Baron Miinchausen?

Yes, there a real Baron Miinchausen.

Baron Karl Friedrich Hieronymus von Miinchausen (1720-1797), a German adventurer, is believed to have served in the Russian army against the Turks.

He was known for exaggerating his exploits.

Satirical stories about him were told by Rodolf Erich Raspe in Baron Miinchausen, Narrative of his Marvellous Travels (1785).