[kayoone]

Is that how americans say it?

Sounds funny and like a direct translation from German: Einem geschenkten Gaul schaut man nicht ins Maul.

[alex_stoddard]

Earliest known version of the saying is Latin "Noli equi dentes inspicere donati" from Saint Jerome around 400 CE/AD. Most European languages are going to have a version. I agree the modern English is peculiar, it likely derives from Middle English. Would that be closer to the German?

[X-Istence]

Men moet een gegeven paard niet in de bek kijken.

Is Dutch. German rhymes much better though!

[icot]

A caballo regalado no le mires el dentado (Spanish)