Arizona

Maybe it is the longest running. I will now have to go look that up .

I know the Stock Show passed its centennial a couple or three years ago but do not know how long the parade has been running.

edited to add: I found some info on our parade

In Fort Worth, Texas. For over 100 years an event called the Stock Show & Rodeo takes place in Fort Worth's Cultural District. The Stock Show starts off with a parade in downtown Fort Worth. The Stock Show Parade is the biggest western themed parade in the world, and the biggest non-motorized parade in the world. By non-motorized it is meant that no manmade means of motion are allowed, all parade entries either walk, ride a horse, ride a cow, or ride a wagon pulled by a horse or a cow.