I don't know if this has anything todo with horses, but here is a reference from :
http://www.sacred-texts.com/grim/magus/ma247.htmFamiliar Forms of the SPIRITS of MARS.
THEY appear in a tall body and choleric, a filthy countenance, of colour brown, swarthy, or red, having horns like harts, and griffins claws, and bellowing like wild bulls. Their motion is like fire burning: their sign thunder and lightning round about the circle.
Their particular shapes are, a king armed, riding on a wolf; a man armed.
A woman with a buckler on her thigh.
A she-goat; a horse; a stag.
A red garment; a piece of wool; a cowslip.