Unmounted Horse walking rewind
When riding a vampiric horse, and getting off of the mount, the mount starts to wander in a repeated pattern: walk forward, teleport back to where the unmount occurred. Repeat. This also results in the mount not being able to be remounted.
It will eventually slow down to a stop, and then teleport back, where it can then - and only then, be remounted by holding interact.
If resummoned, the bug will occur after unmounting.