I think this is very much implied just by watching the episode:
Ralphie is in “fuck it” mode after his kid is in the hospital. He mentions beating the hell out of someone to get his envelope. He’s clearly willing to do things he would normally hold himself back from
The last scene before Tony meets to confront him about Pie-O-My is where he tells Ralph about how he’s with Valentina now. Ralphie smiles and says he gotta to and then next thing we see the horse is dead. Ralphie clearly took this as “well fuck him then” considering he didn’t lash out at Tony right then and there. And Tony being so fond of the horse is the only reason he didn’t already sell or kill the horse already.