The Frankfurter wurstchen is a German pork sausage dating back to the 13th century. In the 19th century, a butcher added beef to the pork mixture, calling it "frankfurter."
Later it was placed on a roll, and the hot dog was born. We don't exactly know when that happened, but it could've been in 1900 at the Polo Grounds or much earlier on Coney Island.