Cinnamontoast
Fais pas chier!
Contract says:
The loan will terminate in the following circumstances:
either party giving xx days notice; or
breach of the terms of the agreement; or
horse not fit description given by owner or not fit for purpose as set out in clause xx; or
death of horse
Horse is loaned for the purpose of the borrow bringing horse on to event.
So you cover 2 of those re notice/not fit for purpose. I’m pretty sure the vet will say it’s a pre-existing condition. Arthritis is a relatively slow process.
I think the owner is trying to pull a fast one due to not having the horse insured. He/she can demand the moon from you, but ultimately, I think the vet and the law will side with you. I would ask the vet for a written report and return the horse. This will probably need organising and paying for by you. Try not to let the owner intimidate you.