Nederlands: Een gegeven paard kijkt men niet in de bek… 
Edit. Oops. ‘Bek’ must be “mond”. An ordinary animal has a ‘bek’ but a horse is a special type that has a ‘mond’ or ‘mouth’ in English. Sorry horses, typo! So the correct Dutch saying is: Een gegeven paard kijkt men niet in de mond.
Oh, my! What a language!
I feel you have to endure a really CrudeTherapee – or at least a RawTherapee.
Gift analogy is a good one.
And the saying, in Finnish: “Ei ole lahjahevosen suuhun katsomista”. I did some googling and found the article below if someone is interested of the origin of the phrase.
As we have most languages of the world by now, here it’s in German: „Einem geschenkten Gaul schaut man nicht ins Maul“.