Bill of sale, I also have registration papers signed over to me and authorized by the breed registry.
Two of my horses are microchipped and two have passports.
I also have all 4 of my horse's pictures listed under the vets computer system. So that way, an outside party can alwways verify that they have treated the horse, that is the horse in question, and it belongs to me
I come from a tiny town in New Zealand.........and I guess its just never done... you buy a horse you like, pay for it and the deal is done....... I have owned 4 horses and have never come across this , neither has other people I know for that matter.
Altho I havent heard many cases of horse theft in NZ.
Of course it totally makes sence to do it ....but yea ...
2 bills of sale from the agent (one was a hand written one that she provided...and the other was one we wanted her to sign that seemed more "professional")
E-mail from the owner saying the agent had her permission to sell the horse (yeah not a "good" proof, but hey it might work lol)
Registration papers (not in my name of course)
Too many pictures to count and ones with me in also
Two of our three are standardbreds and so we could identify them by their brands. The other is a little pony. We could identify him because he goes absolutely nuts if he is separated from our standardbred mare and he will do anything to get to her lol. All we'd have to do is lead my mare past him and he'd come galloping after her whereas he is hardly interested at all in other mares.
We have all the papers, bills of sale, photos, etc that others have mentioned, but if you're worried about theft, the most important thing is to make sure your horses are in a well watched place. Sadly, with horse theft, the hardest part is usually finding your horse, not proving ownership.