Product becomes defective after purchase do I have a case?

[nose]

I need help with my horse case please from a legally trained person, The horse I bought became defective ( started headshaking ) one month after purchase. I haven't been able to use it as I wanted to because its disturbed by its symptoms. The seller claims it didn't do it in her land, some 300 km away from mine. She says its not her fault the horse has developed headshakers syndrome in my care. I say that it should have lasted longer than one month before it became defective. It was to last years unless it had an accident fior exmaple or developed illness at a time much later. Headshaker syndrome is not something you can cause in an animal, but a new environment with new pollens etc can bring it on because it is related to the allergens it brings in. When I bought the horse they didn't tell me he would get headshaker symptoms in my environment. They say they didn't know he had it. I say that I didn't know he would get it because I knew nothing about headshaker horses till this one, and that my environment would cause it to come on ( not proven) , and naively bought a horse that I thought would perform normally in my environment. Who has the case here , the seller saying they didn't know or me saying he should have performed as its was sold but due to circumstances beyond my knowing or control , he hasn't?