Mischief on the hoof, eh. Very cute.
As you know, I'm a hardware freak and rarely comment on other's photographs, knowing only too well my own limitations, but I'm intrigued by your shot settings. 1/125th of a second isn't really fast enough to freeze motion (see this
shot for an example at 1/500th of a second which has a nice mix of motion blur of the hooves but a pin sharp mane and tail) but maybe you were looking for extra motion blur? I think f/20 is way too big as it offers more depth of field than you need in the shot and don't forget that at anything significantly over f/8 you can get some image softening due to diffraction. A lower f-number would have allowed a faster shutter speed, if that is what you wanted, or a lower ISO number. Obviously, shooting in Tv mode means that to achieve a lower f-number you have to adjust either the shutter speed or the ISO or both!
Cantering slightly off-topic, I notice that you have enabled incremental ISO numbers on your 40D. I haven't been able to find out if the 40D suffers the same issue with this as the 1Ds Mark III but have a look at this article
over at Northlight Images. Not needing such fine ISO control I've left my ISO increments at 1 stop (page 154 of the manual).
I'm looking forward to more shots of her. I love the backlighting around the ears.