you’d need the vet report to be sure but it easily could have been an old or not 100% well horse and just had a heart attack!

I don’t know how much a vet charges for a horse checkup (never having owned one) but a cat is £35 so unfortunately not all owners regularly get their horse checked up. Also the bumpkins tend to offload their less healthy nags for family pets/casual riding, keeping the stronger ones for racing or draught work. So this one could have had any amount of existing unknown underlying problems…

there was a horse for sale in the pets corner section of my local free paper!