El dicho en inglés: "Don't look a gift horse in the mouth."
What it means is be grateful for the gift no matter what.
You can tell the age of a horse by his teeth. The older the horse the more inclined the front teeth and the more worn out the molars. It refers to being grateful you at least received a horse to travel on, no matter how old.
In Puerto Rico we say: "A caballo regalado no se le mira el colmillo".