This idiom is pretty straightforward. And it doesn't seem to have an origin that has evolved. It has always carried the meaning to hold on and practice patience. You can read about when it first started appearing in print and other media here .

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, that is, stable lads, trainers etc. The notional 'from the horse's mouth' is supposed to indicate one step better than even that inner circle, that is, the horse itself. It is a 20th century phrase. The earliest printed version I can find of it is from the USA and clearly indicates the horse-racing context - in the Syracuse Herald, May 1913: "I got a tip yesterday, and if it wasn't straight from the horse's mouth it was jolly well the next thing to it." ( source ) Today it means to hear something from the highest authority on the subject.

The origin of this common phrase is unknown - it just kind of appeared one day! You can read about it's appearance here . It's meaning is don't be ungrateful when you receive a gift.

Although there aren't any confirmed sources for this incredibly popular phrase all the theories circle around sports, of course! You can read about the varying ideas here . 

In horse racing sometimes the jockeys beat the horses to go faster and that's where this phrase originates. A horse who is worn out can't go any faster no matter how much it gets beat. It is pointless. So to beat a dead horse means to bring up a topic that cannot be changed or concluded. ( source )

Dark Horse is a term that was born in horse racing circles.  Click here for the origins of the term and for uses of it. (And watch the video - fun!) These days it means anyone who comes up from behind or out of nowhere and takes first place, so to speak. For example, my current job position. I was called the dark horse by getting the job offer. They had already done several interviews and were pretty much at the end of people and ready to make a decision when my application came in and they decided to do one more interview. I got the job. *wink* Came in like a dark horse, ha!