I agree with Eco on this one.
Caveat emptor.
Whether the horses are there legally or not, if you're taking advantage of the beach, watch your back.
You keep your eyes on your kids when they're swimming, right?
This seems similar.
To me the local issue would seem to be sanitary over anything else.
Unless a horse is wearing diapers . . . we're all grown-ups, we all understand a horse's capabilities in the defecation dept.
Furthermore if you are going for a horse ride, this is a horse - not a car, there isn't a gas pedal nor a brake.
If you're not an accomplished horse-person don't get on a horse you know nothing about.
Believe me horses can tell.
For no fee what so ever, I offer the same advice about scooters - road rashes seldom enhance vacation.
As far as throwing something at a horse . . . hmmm?
That seems like something that in the long run isn't going to achieve your desired results (which I assume is having a fun, relaxed time in the capitol of casual).
Besides you'll probably hit the horse and it sure isn't their fault they got dragged onto the beach.
It's Negril, why waste a lot of energy on being uptight?
If you're looking for a hermetically sealed vacation destination I understand Disneyworld is completely safe and entirely scrubbed free of real world dangers.