Me needs a Magical Horse for a militant Hoplite magus. I lack an inspirtion to make this creature cool and interesting. Currently, I'm trying to compile something from Aeolus (RoP:M), Stallion of Camargue (RoP:M) and Bahild (MoH) but I would like to see something else too. I'm particularly interested in Simo, warhorse familiar of Rutgiers de Gerberoy from L&L, maybe someone has some unpublished material that could be shared?
Do you want a horse suitable for fighting, dealing with mundanes, or dealing with magical enemies?
Possible power sets:
Fighting: Shapechange - size increase, claws, big teeth, armoured, full dragon shape, wings for flying, etc.
Mundane influence: leadership powers, to control/influence mundane horses, lead other animals, looking impressive, improved int, shapechange to small animal for spying, scouting (flying?) eg cat, skill bonuses.
Magic: improved magic resistance (more might), or offensive? eg fire breathing, plus the fighting set.
Have you ever read the Dilvish the Damned stories by Roger Zelazny? He had a magic horse that knew how to do mid-level ritual-type magic, banishing demons, detecting magic, etc. Dilvish at first thinks that his horse is a demon and has to make a bargain with him in order to escape from Hell; at the end, he discovers that he is possibly an angel. Either way, he is made of metal (armoured), is very intelligent, can do all that a normal horse can do (but is much heavier), and can cast magic spells, albeit slowly. He is also very sarcastic about Dilvish's own magical ability being weaker than his own, and very knowledgeable about past history.
Gilarius.
I've never read Dilvish the Damned but now I think that I'll definitely try it. Anyway, the idea of magic-using, demon-banishing, divinely-tainted horse is just great! Perfect for pious Flambeau magus. Thank you, sodales 
Or wait for Marko to post his magic bridle that can be used on any horse. Much more affordable alternative while you work towards Dilvish 's horse.
Amphetamine would probably be more useful?
That would be dedicated Muto Mentem vis... for sure I need some to develope a story for such a creature... 