After research, I found that the term "gift horse" means this...
"don't be ungrateful when you receive a gift"
Here is a further explanation...
As horses develop they grow more teeth and their existing teeth begin to change shape and project further forward. Determining a horse's age from its teeth is a specialist task, but it can be done. This incidentally is also the source of another teeth/age related phrase - long in the tooth.
I know this sounds crazy but at least you learned something!
I'm backing up the Brink's truck on this one...
Basketball - 535 Wagner +15 -105 for Game
Good Luck Handicappers!