I made some very low-flying units for RTW for my 'Metal Mayhem' mod. I made a new animation set that had a rotating set of rotor blades, then hung a small droid thing underneath the rotors. The blades were both the 'flight' mechanism and the weapon

They flew just about head height, so the enemy attacks looked natural. Anything higher, and the attacking melee weapons are aimed in the wrong place. I kept them small, so that the creatures did not look out of place hovering at that height. Make the thing big..like...say...a Pegasus.. and at that height they do not look like they have bothered to fly! Make them small...like a small imp or devil, and they look much better. Presumably, increasing the horizontal component of the animation would help too. Make them move faster than a galloping horse and you also increase the illusion of flight. I did that too for my hovering drones.

You also need to be careful with the death animations as well You need to make sure they actually hit the ground properly!