Put the summary of the bug you're having here

What I expected to happen was...:
attempt to cross creek, be dismounted once horse sank enough, get back on horse, move further across the creek, be dismounted, get back on and finish crossing creek

What actually happened was...:
attempt to cross creek, be dismounted, horse disappears, save and exit, re enter world, get back on horse

Steps to Reproduce:
Be forceful dismounted off horse due to horse sinking to far in water (1/15 times)

P.S

It is unknown to me whether willing dismounting a horse will cause it to disappear