This is the time to say your good-bye. When the vet administers the final injection, stand aside and out of the way, because your horse will fall hard and fast, as death is instantaneous. Again, prepare yourself for this, as it is one of the hardest moments to witness. Many vets will not allow owners to be present during the final moments, so if you want to see this through to the end, hold your emotions in check until it is all over. In my experience, the dead body of a fellow horse in the barn overnight was not traumatic to its other stable mates.
Animals deal with death differently than people.
If the death was a violent or savage act, they would respond differently. If you have only one other horse and it was very attached to its stable-mate, expect the same traumatic response as you would have if you sold the horse or otherwise separated the two. In some cases, the horse will emit an involuntary gasp minutes after death. This is NOT a sign that your horse is still alive or is suffering. On the same note, sometimes muscles twitch or eyelids flutter.