Bug:

This seems to be related to, but distinct from, other "disappearing mount" bugs. When riding a horse, if I enter third person mode, the horse will become invisible and have no hit box upon dismounting. The only way to have it reappear, is to exit to the title and reload the world; reloading chunks with f3 + A does not cause the mount to appear.

Reproduce:

This happens consistently, whether the third person camera is opened while on the horse, or before mounting the horse; and, the third person camera can be opened or closed when dismounting, and the bug will still occur.