When I worked in a forest in Panama that was heavily infested with ticks
sulfur powder was a must. An additional technique many people there used
was to tuck pant legs into tall socks, and wrap a wide piece of masking
tape over the top of the sock, sticky side out.
If ticks are a problem for your horse, you could apply the sulfur powder
and masking tape to your horse's legs too. Perhaps on top of polo wraps.