what type of cancer? I knew a horse who had skin cancer and lived happily for a couple more years, it was not spreading. If your horse's cancer is certainly going to cause him a slow and painful death, then yes i would advocate putting him down. Sadly hard decisions like these are part of horse ownership, and it is up to no one but you. If there is no way to cure it and it will only get worse, then i would say it's only kind to euthanize your horse. I know what you are going through, but you have to think of what's best for your horse. Maybe talk it over with your vet and ask specifically how bad it is if you haven't already. I'm sorry for your situation, and i hope you find the best solution for you and the horse.