Humane Killers have been around since before the turn of the century ... but you do have to be in immediate proximity of the animal to use it.
They don't shoot horses, do they ... but it sounds like it. The device operates by using a .32 cal blank round and the force of the contained concussion kills the animal. I have also heard of versions that drive a spike into the animal's head.
Edit to correct laugable spelling mistake.