This is difficult. I assume there was no formal loan agreement put in place?  Unless you have something in writing to state ownership then the owner can really come back and take them whenever he wants.  Also there is the question of the horses' passports, who has them?  Legally the passport needs to accompany the horse travelling etc so if you are in charge of the horses then you should have the passports, as a DEFRA representative can ask to see them at any time, even by simply walking into your yard, and if you don't have them you could face a rather large fine of thousands of pounds.  You say that the owner has been telling people he has sold the stallion so presumably if you can't get hold of him then maybe one of these people he has been talking to will know of his whereabouts and can put you in touch.  Really without any formal paperwork/receipts you can't prove ownership.  I would seek formal legal advice, at least to help get hold of the owner.