Will I hurt my horse if I use my basic ThinLine pad directly on my horse’s back?

No, you won’t hurt your horse, but the longevity of your basic pad could be compromised. Still, most riders see close to a decade of use out of a ThinLine pad. Just be sure not to pull the pad up at the wither into the gullet of the saddle. Though you may see it sit on the wither, it will not cause a pressure point.