If I cast Greater Steed and fly straight up directly over my foe, can I get a cover bonus to my AC if the foe attacks me with a ranged weapon?
From PHB p196:
Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
The steed certainly counts as a creature, and while you could squabble over the linguistics of the word "side" (having something between you and your attacker on the vertical axis instead of the horizontal one could arguably not count as a "side"), I think the written intent is clear. There is a horse between you and the thing trying to shoot you. Some amount of cover should be granted.