It’s very normal for young horses to mouth and play with the bit. They’re just trying to figure out what the new piece of equipment is. When I put a bit in a horse’s mouth for the first time, I like to practice groundwork with the mecate bridle so the horse gets used to feeling the bit in his mouth, and at the same time I’m getting him to use the thinking side of his brain by moving his feet. That way, before I get in the saddle and pick up on the reins, he already knows how to respond.
At the end of the training session, if the colt is still playing with the bit, I remove the reins and let him get comfortable wearing the bit. I turn him out in a safe area, where he can’t get the bridle hung up on a fence post, etc. I’ll even let him eat in the bit and drink with it. Basically, I just let him get used to feeling it in his mouth. You want the colt to think that the bit is just another part of the day, not something he needs to worry about or get fussy over.
Sometimes when you first put the bit in a horse’s mouth, he’ll get his tongue up over it or stick his tongue out the side of his mouth, etc. I just ignore all that behavior. If he was able to get his tongue up over the bit, he can figure out how to get it back under the bit. If he wants to stick his tongue out the side of his mouth, that’s his business. He’s got to figure out that no matter what he does, he can’t get rid of the bit in his mouth. I want him to get so comfortable wearing it that he thinks that it’s just a natural part of the day. I always tell people when they’re starting colts and introducing the saddle that they have to make the horse think that the saddle just grew out of his back like his mane and tail. The same concept applies to letting a horse get comfortable wearing the bit.
What you don’t want to do is put the bit in the colt’s mouth, see him mouthing on it and then immediately remove it, because that just teaches him that if he reacts to the bit, it’ll go away. You want to teach him that the only way you’ll remove the bit is when he’s relaxed. Usually, after a week of consistently wearing the bit, the colt will stop playing with it.