Horses/mules are taking damage when dismounting after being ridden. The longer the ride, the more damage they take. The first occurrence happened after a long ride and the horse died immediate upon dismount. Shorter distances cause less damage. Mounting and then immediately dismounting does no damage at all.
This is newish behavior as the first horses that died were obtained either in the last snapshot, or the snapshot before.
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In my experience, this appears to be related to jumping the horses while riding. Seems as though they accrue fall damage that hits when you dismount.
I've been killing horses for a while this way. Does not seem to be dependent on jumping as just a long ride will do it. More, jumping does not seem to work. Instead of jumping, the horses just neigh and rear back even when at full trot, and do not jump up at all.
Jumping works for me, however, I have tested more and climbing does cause the same issue, albeit slower to accumulating. I have not been able to cause the horse damage when riding on level ground, yet.
Going up and down hillsides also causes stored-up damage to horses. Even if they are just single block descents. Just took a donkey up the side of a hill and back down the same way it came. sidestepping so that it went down slow, and dismounting took most of its health.
The temporary way to prevent this is to dismount horses into water so they don't receive the accumulated damage. But with this problem I've arrived at the point that I died with my horse while going through a plain biome because of yes.
As you guys said is like while jumping (even less than 3 blocks of height) or going fast with the horse, it accumulates certain damage (the longer it takes to dismount, the most damage the horse takes).
I can confirm this, was wondering why my horses were dying when I parked them in their stalls, tested it multiple time, to reproduce grab a horse, ride it for awhile and park it, they take serious damage and some die instantly if they don't have enough hearts.