Mobs are able to step up automatically while falling
in 1.8 when pushed by slime blocks (sideways) a entity is able to jump up over a one block gap and up by its automatic step up height
in 1.9 this has been increased to two blocks (shown in v=1VaMGOYdSX8&feature=youtu.be )

For most living entities (inc the player) this can jump up by half a block onto a half slab above,
however for Enderman and Horses this is increased to a whole block (again shown in v=1VaMGOYdSX8&feature=youtu.be )

Summary Table

The following table show how far vertically and horizontaly when pushed by slime blocks (sideways) a mob can move, Horizonally is measured as the air gap inbetween the blocks the entity move from and the block the entity moves to, and vertically is the number of blocks the entity can move upwards above the block the entity was pushed from, NOTE: none of the mobs where jumping at the time of testing (especially the player))

The way to fix this bug would be to disable the automatic step up code whist the entity is falling / not on the ground (like when flying in creative mode)

I'm sorry if this report don't make much sence it's a little hard to explain though

EDIT

This bug is not a Duplicate of MC-89030 as it can be shown in ' https://youtu.be/Wb0GfHunrRA ' to also affect 1.8.9 which is unaffected by MC-89030

EDIT 2

Added an world where you can automatally test if the bug still affect that version (called 'Bug test World.zip')

EDIT 3

Changed the bug report from only effecting enderman to affecting all living mobs, would like to thank panda for his help suggesting horses as another mob affected by the bug)