Ok, Here's the deal. I have an 18 month old 15h yearling Huntseat filly who as of a few months ago decided she was going to start pulling a pretty clever stunt. I lunge her and at the walk and trot both ways she's fine, perfect angel, than I ask for her canter, she canters about 2-3 laps (at the most) than takes all of her body weight and does a quick 90 away from me, ripping the lungeline out of my hands. Only once have I ever been able to hold on and stop her from going. She even pulled this stunt last week at the trot. I couldnt believe it the first time she did it. Yesterday I was bringing her in from the pasture for the night, another horse in the paddoc as we were walking by was running up and down and my filly got excited, usually if she's on a regular lead, she is controlable. Not yesterday. She started running sideways and as soon as she got her head facing away from me she took off running like a complete idiot. I don't know what to do. Its getting worst. She's not heard bound to any horses or anything. When I lunge her, she takes off and runs to the other end of the arena, and either stops, or she runs to the other end than runs back to me. I keep her at a huge boarding stable and Now I cannot lunge her with other horses in the arena as I don't want her to pull her stupid stunt and get away and piss someone off..... We have a big round pen, but due to living in oregon its now sopping wet and not safe. Any ideas?