STRAIGHT FROM THE HORSE’S MOUTH
Some knowledge received direct from the highest authority, from the person whose word need not to be doubted.
The expression comes from horse racing, where the tips to be trusted came from those closest to the breeders [one who raises animals] and trainers.
The phrase implies that you’ve heard something from the best possible source-in this case, the horse itself.
A variation on this as a source is the idea that the true age of s horse can be ascertained by an examination of its mouth. The first permanent horse teeth appear in the centre of the jaw at the age of two and half. A year later, a second pair appears, and at between four and five years, the third pair appears.
Therefore, no matter what an owner may say about a horse’s age, the evidence is in the horse’s mouth.
(Taken from Judy Parkinson, Spilling the beans on the cat’s pyjamas, Michael O’Mara Books Limited)