The best riding pants I've found were designed by arborists as a hybrid between standard canvas pants and rock climbing pants. Meet Arborwear's Original Tree Climber, made from 12.5 ounces of prewashed canvas that's soft to the touch, and tougher than hell.
What makes these pants so ideal for riding is the unique Arborwear cut in the crotch and contour of the pants. The fit keeps them in place and allows you to lift your leg higher with much less pulling. The cuffs are tapered so they won't hang on obstructions and you can actually get them inside a pair of rubber boots-- something I can't do comfortably with most Carhartt's.
The knees are double layered and the pants are truly industrial stitched. My five-year-old, first pair of tree climber pants is still in service and are just now showing signs of knee wear. I still use them riding, bird hunting, and working in my firewood lot.
There are other designs in the line that shed water, weigh less, and can be worn for casual dress applications. However, when it comes to comfort and protection from errant limb slaps while riding hard, the Original climber pants are my go-to.