Re: OFF-TOPIC: resource for sayings

["neha ." <snehasn -at- hotmail -dot- com>]

Mike,
Thanks for putting the question out there - it's a fun one!

Try http://members.aol.com/MorelandC/Phrases.htm. Click on the Phrases with 
Origins link in the left panel. The following origin for "Straight from the 
horse's mouth" pops up:

*Meaning: Directly from the source.
Example: If you want the real story you have to get it straight from the 
horses mouth.
Origin: This is a boast of confidence from a racetrack tipster, who says he 
gets his information from the horses themselves?thereby assuring the bettor 
that the info is the correct.
Thanks to Jim Hubbell

Alternatively,

A horse trader would bend the ear of a prospective buyer with all kinds of 
talk about the animal, but for a clear measure of its worth, one can simply 
look in the animal's mouth. You can tell a great deal about a horse from its 
mouth. Age, nutrition, general health of the horse, and if it had been over 
reined.

If a horse is unruly you have to rein it in a lot, and this shows in the 
horse's mouth.

Thanks to Gregory Pius, Radar, and J. Gold

See the related phrases Long in the Tooth and Don't look a gift horse in the 
mouth.*

-Neha.