Horses can be pushed into a block on dismount. I've recorded how to reproduce the bug and uploaded it to youtube.

https://www.youtube.com/watch?v=6lsBDwz-hAI
edit: new video with hitboxes: https://www.youtube.com/watch?v=TUeio69qO6s

What's happening in the video:

• I spawn a horse and push it out of the wall
• I mount and dismount the horse
• The horse is pushed into the wall, takes damage and dies

This can also happen in bigger holes, but it's harder to reproduce.

I don't think this is a duplicate of MC-96583 because there is no elevation involved (if it is in fact a duplicate, the bug is clearly not fixed)

Some people reported this behavior before (eg: MC-97243), but the trail always leads to the above mentioned elevation bug.