It’s usually accidental external injury cover on veteran policies - ie. Horse put its leg down a rabbit hole (witnessed not guessed/assumed) or got a wire cut.
You can try, especially if the horse is retired but I wouldn’t hold out too much hope personally, sorry
Hes a companion pony so not ridden anyway. We are guessing he has injured himself in the field. Vet is not really sure what it is yet but guessing ligament.