I'd vote for call the local media at this point, too. The police could be short-handed right now with budget cuts and have put your case in the Priority Z bin. A call or two by local news reporters can sometimes shift their priorities a bit.
Go get the horse! And LET him call the police! the more time that you wait to act the BETTER it looks for him that you abandoned the horse! Go get the horse..LET him call the cops...And even if you cant get the horse back right then.. the police will HAVE to file a report on the situation then....it will give you MUCH needed documentation that you are indeed TRYING to get the horse back... I'm sorry this is happening to you!
Since he broke the contract you should be able to go to the courthouse and get your repo papers. Hire a off duty deputy to serve them and get your horse back. I used to charge $50 to repo for personal property, that was 30 years ago. So it may be more like $100, whatever it is file in small claims for the cost of the repo, after you get your horse back.
Note some bail bondsmen will do repos too, but they will be much more expensive than a deputy that needs the cash.
If this is a civil matter (as the sheriff et al keep saying) then I really don't see how going to the uncle's place to remove your horse will suddenly become a criminal matter.