My horse did something similar. He had broken his right hind leg terribly in several places, and my vet didn’t want to tranq him because she knew he’d struggle and fall the way his leg was (it was horrible). She knew how much I loved the horse (he was 22 and I’d watched him being born…love of my life), so she sternly sent me away because she knew that he might not go down easily…he was very healthy except for the leg. So, I went into the barn and cried with my parents. My dad eventually looked out and said, “He’s down.” I waited a minute, then looked out and the vet was checking for a heartbeat. She called to me and said I could come out.
As I had just taken hold of his halter to remove it from his head, he gave one of those big sighs. Scared the hell out of me. He did another one almost immediately and then was quiet. The vet was so apologetic. Said she didn’t expect it because he actually went down really easily. But she said it’s a reflex, that the lungs are still attempting to do their job.
@Grey I am so terribly sorry about the loss of your boy, and especially how traumatic it was. Horses are big animals with very strong survival instincts, so while it may not be common for such a reaction, it’s certainly not unheard of. I doubt your horse was really conscious of what was happening, and it was almost certainly much more traumatic for you than for him. Don’t beat yourself up. You made one of the toughest choices we have to make as horse owners, and it was the right choice. He’s at peace now and feeling no pain.
ETA: Not sure if this will help…but think about chickens running around after their heads are chopped off. It’s because their nervous systems and physical reflexes are still operating even though their brains are no longer attacked to their bodies. In the case of a horse still “running around” and physically flailing during euthanasia, it might be similar? His brain had probably checked out, which means he wasn’t mentally aware of what his body was doing. I know it’s still traumatizing to have seen, but I think it’s easier to handle if it’s just physical reflexes taking over, not the horse mentally fighting the situation.