I'm in the camp of removing the camera and leaving a contact. After all you "don't really know who the camera belongs to." If the camera is on your side and the stand on the neighbors side maybe "you thought somebody might be trespassing on both properties"

If it is the neighbor that is how I would start the conversation.

"I didn't know who's cameras it was but I found a camera and was concerned about my horses" maybe you "don't mind them putting their camera there and are just concerned about shooting/hunting on your property with your horses"

Gives you a chance to feel it out without making accusations or being a pushover.