I had a Quarter pony mare once who CONSTANTLY lipped/mouthed/chewed/chomped anything in or near her mouth. What I mean by "near" her mouth is she has been ridden in also a hackamore and she would reach sideways with her lips and try to grab the shank to pull it into her mouth. When she did have a bit, whether a simple broken snaffle, to a broken curb she was always chewing, mouthing, or moving it around in her mouth. There was nothing wrong with her teeth, she just had the habit of messing with the bits and if it wasn't in her mouth she'd grab it to put it there. It was rather amusing.