Basically saw this during two worlds I played on, one without Optifine and the other with Optifine. While using ctrl to speed up the already fast horse it gets stuck at a difference of level and the animation glitches. As soon as I release the forward key it automatically teleports on top of the block. I think it may be the rendering or the speed is just too fast that sometimes it is unable to climb from y:62 to y: 63 and it gets stuck in between. The game runs at constant 90-120 FPS with everything set on high. No lag, crashes or disconnects during the bug.