In horse racing circles tips on which horse is a likely winner circulate amongst punters. The most trusted authorities are considered to be those in closest touch with the recent form of the horse, i.e. stable lads, trainers etc. The notional 'from the horse's mouth' is supposed to indicate one step better than even that inner circle, i.e. the horse itself.
It is a 20th century phrase. The earliest printed version I can find of it is from the USA and clearly indicates the horseracing context - in the Syracuse Herald, May 1913:
"I got a tip yesterday, and if it wasn't straight from the horse's mouth it was jolly well the next thing to it."
This is not at all correct. In the past, a horses liniage was listed inside it's mouth. Tattoo'd if you will. Everybody tried to lie about a horses liniage, to get a better price. Once it became practice to put the info in the horses mouth, one couldn't get away with it anymore. So when asked about a horses liniage, one could say, " I got it from the horses mouth...". This is also the etomy of the phrase, " never look a gift(ed) horse in the mouth..." meant to mean, that you don't check the horses mouth for liniage, when someone gifts you a horse. This would be rude. Sorry, but the previous could not be more wrong.
kissing
The expression is "Don't look a gift horse in the mouth." People who buy and sell horses, can tell the age and health of a horse by examining the horse's mouth. However, if someone gives you a horse, you should not examine its mouth to determine its value, simply accept the gift. So the expression means, "If someone gives you a present, do not try to determine what it is worth. Just accept the gift and say, thank you."
There is probably no connection to the expression HEY! and the feed for horses. The Hey is an exclamation or surprise expression.
It is a possibly, but there are no reported cases of it in horses.
A plain snaffle or a happy mouth.
A "bit" is a metal link across a horses mouth used when riding. The term normal means that someone has a grasp some information on something and is following it through
The phrase "healthy as a horse" means to be in excellent health or physically fit, typically used to describe someone who is robust and strong. This expression is drawn from the perception that horses are generally sturdy and resilient animals.
Word of mouth is just and expression used to advertise or publicize something on a person to person basis. If something is traveling by word of mouth it typically means it is traveling like a wild fire, and that is fast.
fod
Their mouth....
A horses bars are the toothless spaces in it's mouth, where the bit is placed.
A bit is metal bar that is on the bottom of the bridle. This fits inside the horses mouth, behind the teeth, where there is a gap.