I had a naughty mare once...much worse than this pony. I rode her in open hunter on the flat classes, and if I was not careful, she would take off at a run, head straight up in the air, not caring one iota where she was going. She would also do this when I rode her in the pasture on occasion. Scarey, scarey to have a horse run without thought as to where it was going.
I finally discovered that the bit I was using was hitting her in her soft palate whenever I took ahold of the reins to any degree. It taught me that naugtiness often has underlying causes other than the creature is out to kill me.