A Lightning Lord would appear riding a small cloud. When the player gets close, the Lightning Lord tries to hover directly above them. Eventually he'll stop, the player will hear the sound of electricity charging and after a while a streak of lightning would come from the cloud straight down. The lightning bolt leaves a 1*1*1 crater behind - the space in this crater is on fire. Lightning bolts go straight through leaves, setting them on fire.
When a Lightning Lord is killed (preferably with arrows), it's cloud remains. A player may be able to jump into the cloud and ride it for a moment. Movement in a cloud is much faster than on land, but because the player is heavier than a Lightning Lord the cloud slowly goes down. When it touches the ground the cloud disappears. The player can also jump out of the cloud at will.
Lightning Lords will only appear up near mountain areas. Floating maps will be swarming with them.
C+C is nice.
EDIT: It's spoils would be a feather/gunpowder, as well as it's cloud.
You probably can't tell, but I designed this guy like the default player, except in super pissed off mode. After all, if lots more mobs get added we're going to need some more humanoid ones.
Secondly, I thought he could be a dangerous daytime mob. Now before you moan, remember that this guy is only really going to attack you if you're high up, so unless you like building huge towers you're not going to see much of him. Besides, it'd be easier to see him during the day, which means you won't be randomly flinging arrows up in the air. Being a daytime mob he disappears during the night.