This ticket does not seem to be a duplicate of the referenced one. MC-14165 discusses a visual glitch in which the lead does not disappear despite not technically being attached. This ticket discusses 1) horses being capable of flight when the player is far enough above ground level, and 2) horses acting as though the lead is still attached when flying even though it technically shouldn't be and does not appear to be. I'd like to add the fact that when returned to ground level, the flying mob will still take the appropriate amount of fall damage as it would have if it had fallen from its maximum height.
The screenshots fail to show that the horse moves seemingly randomly on all three axes at a speed much greater than it would at ground level.