This one comes from a website called Know Your Phrase (www.knowyourphrase.com)
"Never look a gift horse in the mouth! Wait, why would I not want to do that? Well, think of it this way: if someone was polite enough to give you your very own horse as a gift, would you then inspect the animal to see if it's of good quality, especially when the person who gave the horse to you is standing right there next to you? Probably not, because that might be seen as very rude behavior.
Apparently, though, there were people who would inspect the horse, particularly the mouth region, in order to determine the horses age. Yeah, you can do that. The length of a horses teeth can be used as an indicator for their age; the longer the teeth, the older the horse."