I did a lot of riding in my youth. Years later, I considered horse skidding, back when we decided to log our property ourselves. The practice got a passing mention in one of my WSU sustainable forestry courses. I know someone who did this for bug-kill removal in Idaho's Sawtooths. But we didn't try it, in part because we didn't know who had horses and rigging for the job.
"King County turns to horses to move logs through delicate site" (Seattle Times 9/22/10) describes a county biologist's use of horses to move logs into sensitive creekbeds. She was rebuilding fish habitat, so heavy equipment would be counter-productive.
If you're really interested in learning about horse logging so you can use it on your place, contact Andy Perleberg at WSU -- or find Wes at his company, Wood'n Horse.