For the sake of being on topic for the thread I have euthanized a “pasture sound” horse.
I bought a 10ish y/o kill pen (undisclosed) mare off a very unethical seller (yes, no PPE). They knew she had feet/teeth problems because she took off a little at the last minute because the teeth/feet needed to be done. The first thing I did was address the broken half rotted tooth, broken and abscessed jawbone. She was plenty happy in the pasture and she was fine as long as you didn’t push her too much. She would meet me at the gate, follow me around, generally be a sweet in your pocket mare……in the pasture. Happy to run, buck and play. She tolerated an adult grooming her and basic vet work, but any kind of intensive issue and she would strike and rear. Once she threw the twitch across the room.
After about 6 months she got tricky to do her feet. At first I blamed the farrier because it started after he hit her in the face (where the broken jaw/tooth had been) with the rasp. But after a few more tries at trimming her she required sedation. If you did not sedate her she would pull back.
I eventually got her into a trainer (same one who wanted to do EPM treatments on my current horse). After a week she wanted to X-ray the feet. The vet described it as “holes” in her navicular bones and would have euthanized right then. We could try “stuff” but was doubtful it would help. I decided to try stuff and had the trainer move to focusing on ground manners. The next week the trainers kid was grooming her and for no reason she went up and over. I made the appointment the next day. She was absolutely the sweetest thing on earth that day, even letting my kid give her all the attention she didn’t normally allow and she fought like hell when it came time.
The horse was absolutely fine, and could have been managed, in the pasture 98% of the time. But that 2 %, she was very clearly miserable.