I'll try to explain:

Hiro only dreams about being raped by the horse. Hiro is the older brother, who used to be sexually abused by his father. His father raped him and fisted him. His father probably raped him out in the barn, where he kept his horse, though I don't think he actually had sex with the horse. I think because of the trauma, Hiro sees his father and the horse as one, since being fisted and a size of a horse's dick is probably similar. When his father dies, Hiro goes home for the funeral. Eishi is the younger brother, who accuses Hiro of being the one his father loved the most (because he used to see what happened). Hiro get's pissed, telling Eishi "that's a lie", "what do you know" and that their mother even knew about it but didn't do anything. Hiro being pissed, takes it out on his brother and sort of starts to rape him - holding him down and forcing Eishi to blow him - but Eishi starts to reciprocate by fingering his ass. Eishi tells Hiro "you're not raping me, I want to do this" and also starts to fist Hiro. Eishi tells Hiro he will be the replacement for father. The end.

I think what was most traumatizing for Hiro is that he enjoyed the fisting. I think the horse is definitely symbolism for his father - a kind of cross over from being fisted and raped while being tied up in the barn, most likely in the presence of the horse? It did seem like at one point he was about to be raped by the horse, but I think that was what he saw in his mind. The crazy looking people watching him being raped by the horse in the dream - I think they symbolize the people who knew what was going on (mom, brother, anyone else?), but never said or did anything, most likely thinking horrible things about Hiro. Psych class done.