There may be some legal recourse you could go through for the papers, but then again, maybe not. It's not practical to jump to sue this one, and that one over paperwork. But, as long as you have the bill of sale, that is proof of ownership. Papers are one thing, but bill of sale and physical posession means more most of the time - I'm not saying there aren't occasions where courts find in favor of whomever had papers in their name last, but most of the time, there is a trail of papers - even if it is just bills of sale - that will hold more water than a he said she said p*ssing contest.
I've run into lots of people that will part with the horse, but not the registrations. Usually, they're just nuts. LOL
Ask Often, Demand Nothing, and Reward Generously.