Had you notified the person who leased the horse, and the person who now has it, that you intend to reposses it? You might be able to get them subject to concealment laws if your state has them.
In many states, it is illegal to intentionally conceal property for which there is a security interest (most commonly, this is hiding a car from the repo man when behind on payments, but it could apply in your case as well, since the leasee violated the lease). In many states, concealment is a felony if the value of the property is over a certain threshold, and these clowns might have got themselves in more trouble by valuing the property at "$7,000" earlier.
If you have proof that you tried to take respossession of the horse from the person who leased it, talk to the sheriff and see if moving the horse to the Uncle's place or selling it to him constitutes concealment (if they did this after they knew you were trying to reposses your horse).
I still think you're covered under self-help if you retrieve your own property, as long as you don't breach the peace (i.e. a confrontation, or breaking and entering) in doing so ... but I don't know anything about Oklahoma laws.
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check your local regulations, here is Pa I know you can just call the department and request an officer be present, no paperwork needed just proof of ownership.