Why do some horses run with their tongues hanging out?

It is an impossibility to swallow the tongue-it seems when a horse is nervous then he may try to curl his tongue up in his mouth or control it in other ways-when you find the horse doing this then it is useful to tie the tongue. As far as the tongue hanging out side of mouth-sometimes the tongue is longer than the mouth and doesnt fit in comfortably-the horse may bite the tip when he runs as some horses bite down hard when running and this helps keep it out of the way.

Horses intentionally stick their tongues out like that, actually he horse is not really "sticking" his tongue out. It is common practice in horse racing to tie the tongue to the side for a race. To prevent the horse from sliding their tongue over the bit, in horse racing, they use a tongue tie as I explained before the use of tongue ties.... the horses simply like to fool around with the tongue ties as a way of showing off, as if they were saying 'see what I can do'...lol. :)

~ Edit- What some mean by swallowing is not actually as it sounds, what it actually means- the tongue balls up in the back of their throat, they can't actually swallow their tongues, what happens is if the tongue is not held down so the air can flow across and into their lungs,, so they some call it swallowing it, they don't literally swallow it though...it's a metaphor, similarity speaking. :)

Forgive me for no longer giving an in intensity answer to this question... i've got confidence you have already got 2 remarkable solutions. in spite of the shown fact that, I only wanted to touch on a pair of subject concerns. firstly, I even have one question for you... have you ever worked around horses? the rationalization I ask is because you seem to view "frothing on the mouth" as a severe adverse. authentic, human beings do no longer froth on the mouth, nor do dogs, or another animals. in spite of the shown fact that, I also have a great style of experience using horses... i've got by no skill ridden in horse races, yet in horse shows. while a horse is comfortable, they often play with the bit... this is the piece of metallic of their mouth that the riens are linked to... that's used to instruction manual the pony. while a horse performs with the bit, they make that white foamy stuff around their mouth. So a horse "frothing on the mouth" is fairly an illustration that the pony is enjoying the artwork that they are doing. additionally, the horses intentionally stick their tongues out. to stay away from the pony from sliding their tongue over the bit, in horse racing, they use a tongue tie... that's in basic terms a chew of nylon which you place above the tongue then tie it decrease than the chin. Horses generally desire to fool around with the tongue tie... they stick their tongues out as a vogue of asserting "see what i will do." in case you have by no skill worked around horses, that's common to misread those issues, questioning they're undesirable signs and indicators. yet after working with horses for numerous years, i comprehend that those issues are very easy and not unwell or disgusting. As I mentioned beforehand, i'm no longer likely in intensity to objective to persuade you that horse racing isn't merciless. I in basic terms wanted to in basic terms right the misperception that those horse behavior are undesirable signs and indicators.

Most of the time, you are seeing a horse with its tongue tied. Trainers fear the horse will swallow its tongue.

it helps them get more air into there lungs, i mean if your running for ages and you need to pump loads of air in, you open your mouth to breath?

well when a horse runs it cant open its mouth wide because the "bit" so to clear there passage they stick there tounges out.