Here it's considered 2nd degree trespassing if the ex-bf told her not to come onto his property again, but she does anyway. It's a misdemeanor. She can, of course,go to court and fight the charges and show proof that she has a paid reason to be there, that the horse is hers, ect... but that doesn't ensure she will win or get to take the horse.
Best to keep the dispute in a civil court and not bring on criminal charges too. Either way, she'll likely need to consult an attorney.
I am interested in hearing from our OP as to what course of action the ex-GF has decided to take.