I actually had to put one down for my daughter in law (professional barrel racer). I used my Sig 2340 Blue Line .40 cal. I used two rounds the 2ed just to make sure it was a humane kill.
Big twister went thru our neighbors horse herd, I had to put down eight horses, 12 gauge slugs, no suffering, except me, I don’t ever want to shoot dogs or horses.
.40 works for me when I have to put a cow down. They drop like a sack of crap from about 10 yards (I dare not shoot farther).
They still get a second round, just because I'm paranoid about what it would be like if they weren't dead from the first. Never needed, but it's cheap insurance.
I've killed hundreds upon hundreds of hogs, and had 1-2 get up after receiving what I thought was a perfect head shot. No way do I want that with an animal of mine.