They tend to stick their tongue out the side of their mouth, might suck on it. Generally only while working, but I know an OTTB that does it as soon as anyone so much as looks at him in the paddock! It's a 'vice' caused initially, I believe from mouth pain &/or rushed, superficial training to respond to a bit. Seems it's common in racehorses. I don't think it's harmful at all for them, just ugly & annoying for us!
One of the Andalusian stallions at the farm does that when he's ridden. Before he was purchased and imported from Spain he'd had a head injury so now he's constantly tense in the face so he sticks his tongue out the side of his mouth when we get him to actually relax himself. Poor guy.
I work with a horse that does this, an ex National Hunt racer. He's done it ever since he was in training, pokes his tongue out of he side of his mouth and shakes his head up and down. He does it mainly for attention on the ground, or when he's excited being ridden (does it a lot when he's hunting haha ;) ). When he raced his trainer made his lass tie his tongue down as he did it a lot due to the excitement of the race, as she decided it was ugly, which I felt was pretty unfair. Posted via Mobile Device