HAF, you need to get a receipt for that mare, or something from the previous owners that says Lexi is legally yours or you don't have much of a leg to stand on. Heck, the old owners could potentially say they never sold the horse to you in the first place. ALWAYS get a receipt of purchase! My friend is STILL going through legal battles over her horse... or "her" horse really.
I hate to sound mean about this, but it is necessary for you to get something from the previous owners - march down there and demand something from them, and don't leave their house until you have it.. or write up a bill of sale yourself and present it to the previous owners and all she/he has to do is sign it - make sure there's a witness present to sign as well - preferably someone that you know, not them.