Healthy as a horse?

I started wondering where the phrase "healthy as a horse" came from, I turned to my trusty google, only to discover there were others searching for the answer to this question but most of the idiom sites didn't include an answer. I found lots of links for all kinds of horse related quotes and horse supplements, but nothing on the history behind that quote.

"Don't look a gift horse in the mouth" was a more popular quote, that appeared to have some answers with most believing that phrase went back to the days when a person would look at the teeth of a horse to determine if it was worth buying. So if any of my expert phrasesmith's out there know the answer? I hope you share it.

:-)