>>>The spanish version(s) would be:
>>>
>>>"A caballo regalado no se le miran los dientes" or
>>>"A caballo regalado, no hay que mirarle el diente" or
>>>"A caballo regalado, no le mires el dentado"
>>>
>>>The first one is the one I knew, the other two I just googled, the third one rhymes nicely
>>>
>>>in case you want to start a collection <g>
>>
>>OK, adding:
>>"Poklonjenom konju ne gledaj u zube"
>>"Поклоњеном коњу не гледај у зубе" (the same, just in Cyrillic). And this could have been worded in several other ways (pretty much just like the examples you provided), but this 12-syllable version is the one that stuck. Probably because of the exact Alexandrine rhythm - and now that I've thought about it, most of our proverbs fit into either that, or 8-syllable or (the preferred epic format) a decameter.
>>
>>>>"Darenomu kon'ju v zuby ne smotrjat".
>
>In Russian it is "Дареному коню в зубы не смотрят"

Isn't the horse лошадь in Russian?