Es el mismo dicho, tanto en inglés como en castellano (de dónde habrá salido? ). Aunque yo el de castellano lo conozco más comúnmente como 'A caballo regalado no se le mira el pelo', pero la otra versión también es reconocible.
This old saying implies that one should not question the quality or value of a gift.
The quality or value of a horse is often determined and measured by it's age. A young horse is considered of higher quality and value than an old horse. "looking in the mouth" or "mouthing" a horse is an examination of it's teeth as a means to determine the exact age of the horse.
If someone gives you a horse as a gift, don't question the age or value of the horse by looking at it's teeth - accept and appreciate the gift without question or complaint.
This implies that if you receive a gift from someone, don't question it's quality.