LoveStory, every horse I've seen do that tends to be pretty mouthy, one even bite-y. I think it's a way for them to both release some tension and receive plenty of attention! Haha.
The horse at my barn that does that LOVES having his tongue played with... If you can get him friendly enough with you not to bite in the first place. He rarely ever nips at me now and we're pretty friendly with each other, but I heard he bit a woman passing by his stall a week or so ago. :/ He's cute as a button when he's being friendly and apparently a joy to ride, but pretty dangerous on the ground. I don't think it's that he's fearful, rather he has no respect for most people and is very territorial. His owner isn't much of a "leader" either, and since she's inconsistent with her handling I think it frustrates him. When the all western trainer at our barn handles him he's fine, though she usually have to make him practically think he's going to die the first time he tests her. The trainer I work for would treat him the same way, but she doesn't train jumpers and that's what most of what his owner does with him.
Same thing happens with Chewy, one of the horses my boss trains. He LOVES us, but we think he has more trouble with his owner since she's not a strong leader and he's the kind of horse that really needs that. Concurrent with his barn name, he nibbles on just about anything he can get his grubby little lips on!