Spandex
I guess the real name on those shorts are "compression" shorts. I thought they were made of Spandex but can't swear to it. I thought they wicked moisture but not for sure.
I will also be riding a horse quite a bit. Last year I went horse riding (first time in 20 years) and I thought it was going to kill me. The inside of my legs and my knees got very sore. I thought the biker shorts would at last help with everything but the knees but don't know for sure.