Upon dismounting a tamed horse, while in third person view, my camera movement changes and sort of locks up, only allowing me to look up or down. Pressing A or D also spins my character in place, if I'm standing still, rather than moving my character left or right.
I've asked my friend, and he doesn't seem to be having this issue with horses.
After experimenting a bit, I discovered that switching to first person and back to third fixes this issue. However this camera behavior change always happens when I dismount my horse.