Looks like a Platycotis vitatta but want to check as some of the pictures in the guide look like this, some have different coloration. This one was resting on my car a short distance from the one with no horn on my garage.

I was curious as to why some have horns and some do not, maybe a male/female thing? Perhaps not as I found the article (link below) which says that there are subspecies. Using the key in the article, this one "Treehopper #2, with horn" would be Platycotis vitatta var. quadrivittata? Using the key "Treehopper #1, without horn" (https://bugguide.net/node/view/1527288) would perhaps be Platycotis vitatta var. lineata?

http://entnemdept.ufl.edu/creatures/orn/oak_treehopper.htm

While the treehoppers with horns are trying to look like thorns, why would an Oak Treehopper have horns as oak trees don't have thorns?