Not much debate as to the meaning of this phrase, and two possible origins.
Firstly, the meaning. If a person maintains that his information “is straight from the horse’s mouth,” he is implying that the information is credible, trustworthy and reliable. In other words the truth. In essence this means that the information was obtained first hand, direct from the source or origin.
I could find two possible origins.
The first origin is the more credible of the two. A horse’s age can be easily determined by looking at the teeth. Never tried it myself, but those that know are able to do this. So, if you were buying a horse and you needed to confirm the age, you would open the animal’s mouth, stick your head inside, and check the teeth. Hence, your information would be correct and, straight from the horse’s mouth. Not necessary to rely on a third party opinion.
The second one relates to horse racing, specifically betting on races. If one was looking for a sure bet, you would most probably run into “somebody in the know” who would be able to give you that golden tip. If questioned as to his source, the answer would most probably be “straight from the horse’s mouth”, and no further explanation would be necessary.
As a horse cannot speak, this confuses the situation somewhat. So, my interpretation is that the source is someone as close as possible to the horse, as in a stable employee or jockey, that has inside information not available to others.
Image from Wikipedia
