The phrase 'to hear something straight from the horse's mouth' means that you heard the information from someone who has personal knowledge on the spoken matter.
Example in use: "I don’t believe it that she did it. I’m going to go to ask her and hear it straight from the horse’s mouth."
Interesting fact about To Hear Something Straight From The Horse's Mouth
This phrase is the exact opposite from the phrase "heard it on the grapevine".
