Depends on who is challenging ownership.
In my state, if one spouse takes property from the other, then the police will say “let the family court judge sort out who owns it.” This is because only the family court has jurisdiction to divide marital property.
I don’t practice much criminal law, but I guess this also applies in cases like Cool Meadows’s, where the owner authorized the alleged horse thief’s access to the animal and the property. I think Cool Meadow’s situation is frustrating and heart-breaking and I wish her the best outcome. But I believe I also understand the process she’s having to go through.
If the former friend or trusted employee is willing to forge a BOS, then really it’s up to a trier of fact (judge or jury) to weigh the evidence and determine who’s telling the truth. Police and attorneys can’t do that, and in most cases we really wouldn’t want them to.
So that would be where the indicia of ownership mentioned by other posters - years of Coggins tests and vet records and FB pictures and friends who have known you and your horse for years - would come in. At trial, not before. Unfortunately, in CM’s case, both she and the alleged thief/forger were around when then horse came into CM’s possession. So that makes it an even tougher case.