What is the origin of the idiom "from the horse's mouth" and when to use it?

The idiom "from the horse's mouth" has its origins in horse racing, particularly in the practice of obtaining reliable information about a racehorse's condition and prospects directly from the horse's trainer or owner. To get accurate and trustworthy information about a horse, one would go directly to the source, which is the person most knowledgeable about the horse's capabilities and performance.