As an avid WoW player and fan of the Tyrael's charger mount I just had to do this. The armour isn't 100% accurate (no belly armour) but to make it look decent I had to take some artistic liberties.
The body is based off of Big Macintosh, using the vector made by
Tyrael / Tyrael's Charger is (c) Blizzard MlP:FiM is (c) to Lauren Faust and Hasbro Art is (c) to
I think his cutie mark would be El'druin ( [link] ), the sword of Justice. The sword is what brought his memories back, his identity. I think it's the one item that represents him most. Or something resembling his sympathy for human kind, since he was the sole archangel to defend us.
Well technically I didn't draw Tyrael, I drew the Tyrael's charger (a WoW mount) plus I never played a diablo game in my life at the time I submitted this.