It depends on your definition of what a hotrod, or a street rod is, and who manages to come up with yet another older example. MY def: A hotrod is a fairly m*** production car (that means something there was way more than just one or two production copies of made for sale, and then it was modified after sale to and by a consumer. A street rod is a hotrod licensed for and driven on the street, and not just a trailered race only car. Nothing Henry Ford or any other car manufacturer ever made is a hotrod in my definition because they were all either prototypes, purpose built racecars, or "mules" for product development.
Just for fun, let me post another thought. PROTOTYPES: 999 certainly wasn't a prototype, and I would consider it a hotrod, most hotrods are PURPOSE BUILT RACECARS, and many early hotrods were, Edelbrock, Isky, Brown and most of those recognized as cl***ic examples of hotrods, WERE MULES FOR PRODUCT DEVELOPEMENT
I'll go along with the idea that a hot rod has to be street driven and/or street legal. Otherwise it's a race car. Still, there were plenty of hopped-up, street-driven cars running around long before that funny-looking thing was built. Most were Model Ts, with the motor replaced or modified. My grandfather had older brothers, and they had a Ford with a motor out of some wrecked luxury car back in the 1920s that they used to race rich guys with for kicks. (Gramps has been gone for 25 years, so I can only give you the details I am aware of.) That was in Ohio.
The first hot rod was the second automobile ever made...belonged to the man who wanted to go faster than the first.