When I first got a horse as a little girl we didn't know anything about horses and we always left the halters on. On time my mare got her head caught in the fence trying to eat grass on the other side. She cut her face up a bit but nothing too bad. A year later she got it hooked on the gate and had a huge hole in the side of her face. It healed and was all good. We still didn't learn from that (not very smart it seemed :) ) The next year our 3 year old got his head hooked in the latch for the gate and managed to drag the gate for a couple hundred feet before he suffocated to death. Never ever again. The very first thing my horse learns is to be caught as I will never ever leave a halter on. That being said, if you are working with an untrained, unbroke or wild horse you might need to leave a halter on for reasons that have been mentioned. But not a good idea if you can catch them without.