As with the other topics this quarter, this topic is also an animal-related idiom. It is not necessary to write about actual horses or the things that come from their mouths. Rather, it would be best to write about the meaning of the idiom:
From a dependable source--from an authority--from someone with particular knowledge or insight
The judges are always looking for creativity, and if you take this too literally, you're likely to be marked down for this criterion. Stories about horses, unless they are exceptionally written, will not be particularly creative nor particularly on topic.
It will be far easier to come up with a unique approach to this topic if you write about it with the meaning of the idiom in mind.
To enter the Challenge: http://www.faithwriters.com/writing-challenge-intro.php