A caballado regalado no se le miran los dientes/colmillos:
The correct translation is> Don't be checking the horse teeth when they are giving you the horse as a gift. (IT WILL BE SEEN AS RUDE TO DO SO) The logical thing is, to appreciate what they gave you.
People who buy horses check the horse teeth to see if they are healthy or not, and if the price is worth paying for.
So the saying goes appreciate what you got without questioning its worth bc after all, it was a gift for you and you wasn't the one paying for it.