Gracie always tips her grain bucket over, spilling her grain everywhere. It goes without fail, we have to be careful of where we feed her grain, because it takes her ages to eat it all, lol. Sometimes, the grain falls into a gap in the stall mats, and it's too small for her to get it back, and she gets mad. She throws a fit, tossing her head and spinning around and pawing at the ground. It's hilarious.
That's all I can think of right now, but I want to know what silly quirk your horse has. Please share!