Why do people say "straight from the horse's mouth"?

Search: straight from the horse's mouth

Why: Joel wants to know.

Answer: It's from betting on horse racing!

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.

Like a horse knows who's going to win. As if.

Source: Phrases.org.uk

The More You Know: Speaking of animals and Joel, look who's famous!