Thanks everyone. I really wasn't sure I could make this work and I'm still not sure I 'caught' him. The pose xerox was fuzzy and and with the one showing him more white, I couldn't tell what was shadow and what was coloring. He almost looked flea-bitten (for non-horse folk, that's what they call speckles on a white horse).
I keep a 'morgue file' of horse photos from past portraits, so I wound up looking through that for a horse with a similar build and pose, just to find some detail for the muzzle and eye areas.