This is a good question. If you watch that episode with commentary on both writers discuss this question. One writer stated he believed Ralph did it, the other was less sure and almost leaned the other way. You never get direct confirmation. Best guess though is he did. Why else was he not suprised and not to be found. Also the lack of confirmation is there for another reason. The writer's are indirectly telling you horse murder or not, he deserved death. Killing Tracy, disrespecting the family & the bing, contemplating killing Tony in Season 3, constantly disobeying Tony, and treating others like shit. Horse or no horse his new found spirituality in Whoever did this was just not enough.