This is great thread, and I hope you hit your target - it's a hell of an achievement if you do.
The Doomsteed is pretty cool. I'd use it, but I'd probably ditch Fading Gallop as I think it could drag out the encounter if abused. Maybe make it a recharge, but I prefer monsters to have fewer powers otherwise I tend to forget about them.
Pull the Reins is just a bit nasty, IMO. Personally I'd get rid of this, and add an aftereffect to Nightmare Ride that lowers the target's Will - making it easier for it to use it again.
Nightmare Ride is just a great, flavourful power though. I've seen this in all of your monsters so far - they're all based around one or two really nice schticks. Keep them coming.