Hiya!
Well, IIRC, Pendragon had some stuff for horses...maybe...or was that HarnMaster? *shrug* Anyway, the hand-down, best "horse details" for an RPG that I've ever encountered is in the Hackmaster 4e GMG. You can probably get a copy somewhere for dirt cheep (if you don't already have it), and, if you aren't adverse to...er..."grey legalities"...find a PDF online. Kenzer & Co can't sell their old HM4e stuff (yeah, the first version of HM produced was refered to as "4th Edition Hackmaster"...the current version of Hackmaster is just called that, or sometimes 5th Edition Hackmaster, or Hackmaster Advanced".
But, yeah, HM4e GMG. Not uber-detailed, but has enough info and tables to roll on that a DM can come up with a unique looking horse with a unique attitude and personality in about 2 minutes flat.
^_^
Paul L. Ming