Aperture Priority for the head shots and shots of the horse standing still.
Shutter Priority for the horse moving.
Be sure to expose for the horse, no one will care if the out of focus background is over exposed. Take a few test shots, and if the horse is to dark, then up the exposure +1ev or so, and to light, take it down to around -1ev.
Adjust settings as you go, although i wouldnt want to shoot at over ISO 400 if it could be avoided.
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever