To me, if I see a rule trying to ban "vehicles" without defining "vehicles", I take my concept of "vehicles" by deduction: I think about all the ways that things that I know of as being "vehicle-esque" could be problematic in various different ways such that you'd want to ban them — being loud; having a lot of inertial momentum when colliding with pedestrians; littering (the horse example); property damage (skateboar…
Well, there are many possible options, so this does not help unless you know which issue was bothering the rule-makers.
I know a park that allows horses but not bicycles.