If I remember correctly, he's a Percheron named Tristan, owned by a friend of Tobus's. I remember commenting about that photo as one of my first posts here on Xmarks. I have a love for Perches, so it's one of those facts that stick in my head.

Honestly, I have a personal theory (no research so far) that pants were invented just because of horse riding. Look at the horse cultures of history and we see a prevalence of pants (Native American Plains buckskins, European leggings, Mongol pants...), but if you look at the areas where riding horses was more limited we see kilts and robes (Scotland, Rome, Greece, Saharan Africa, Polynesia....).

So, I agree with the majority. Pants when riding. If you want the jean look, but the comfort of breeches, you might want to look into some riding jeans like are sold here. If it's a dude ranch-type thing with the "cattle drive" thing, pack a PV or Acrylic kilt into the saddle-bags for camp.