Minecraft wiki entry for Development_versions, associated text: NoGravity now works for all entities, not only armor stands.
A mob in midair affected by it, will still respond to knockback by moving horizontally.
Although the AI will not permit any mob to move on their own, horses and pigs ridden by the player, will move normally.
Very similarly, boats in the air ridden by the player, will move as if they were in water.