(Straight) from the horse’s mouth: direct from a (reliable) source.
“I have it straight from the horse’s mouth that Jones Incorporated have had a very good year and now is the time to buy some of their shares before the figures are announced.”
Back the wrong horse: put one’s faith/trust in somebody who later, or something which fails.
“Despite his criminal record I still gave him a job, but I backed the wrong horse because he’s just been convicted of shoplifting.
Trojan horse: apparent asset that turns out to favour a competitor.
“The senior civil servant they arrested for spying turned out to be a Trojan horse, planted by enemy intelligence twenty years previously.”
You can take a horse to water (but you cannot make it drink): you can encourage somebody to do something, but you cannot force him/her.
“I agreed to take the children to the fair, but I refused to ride on the big dipper no matter how they tried to persuade me; after all, you can take a horse to water but you cannot make it drink.”
(From John O.E. Clark, Dictionary of English Idioms, Harrap’s)
