An RPG stands for Role Playing Game. You play in third person as the horse. You create a character, and you control their life. Here is an example - You have created a black stallion called Ebony. Say you took your character to the great plains. You would post something like:

" A stallion of midnight hue trotted lightly across the barren plains. Tossing his tapered, intelligent head he flicked his tail casually and wandered across the plains. His black eyes swept over the terrain, looking for another equine. Seeing none, he reared and whinnied loudly, calling for another."

And then you would wait for another person to post. Say, a day later, someone replied with this:

" A beautiful grey mare wandered the lands. Picking her delicate hooves over the stony ground her ears pricked as she heard the call of the midnight stallion. Snorting gently to herself she trotted quickly over to the stallion, and looked at him, her stormy grey eyes passing over his handsome features."

And then you would reply again, making them have a conversation or something like that.