Mares are very smart, you're just lucky to have one who uses her intelligence for good instead of figuring out ways to get around what you ask
My filly taught herself to tie. As in the first time I tried, and she got to the end of the rope, she stepped forward and that was that. She will stand tied for an hour at a time. Now trimming her hooves, or trotting in-hand... She's always trying to figure out some crafty way to avoid it. But, I can worm her or vaccinate her in the pasture with no halter, and she follows me back to the barn.