First of all, THIS IS NOT THE GHOST/STATUE BUG. I've experienced that bug and it's not the same behavior.

My horses occasionally get stuck in the block they're in. I've noticed it after leashing them to a fence post using a lead and leaving them there for awhile. I can fully interact with the horse and its sprite will move within the block it's on, but it cannot move beyond the borders of that block. I can mount it, open it's inventory, attach and detach leads, etc. It just cannot move from that block whether I try to pull it with a lead or ride it. It just shifts around in that block's space when I try to move it.