I' m not a lawyer, but I think parking on a State highway is a bigger issue than the privacy rights of a horse. Trespassing is an issue if you enter the field. Don' t spook the horses.
And is she beautiful? Just don't let your wife find out ! That is how Trump got into trouble in Russia.
Here in FL our bug eyed hyper Republican Governor made a law that said you could not even take photos of Oranges ... even tho not trespassing on the property. Declared unconstitutional. Suggest putting you hood up...
Common sense. If you may safely park your car on the side of the highway, do so, if a patrolman stops he will usually just ask you to move you car. I have yet to have anyone object to taking pictures of livestock on their property.
Believe it or not I had a photo of a goat rejected by a stock photo agency because I did not have a model release. The goat was in a field by him/herself with nothing around it. Just a goat in a field all by itself. Nothing at all around it. No barns, houses, fences, trees, just a damned goat like a million others. And not a special goat. Go figure. I'd get the release but I don't even remember where I was!