two possibilities for the translation of preterit here:
- il a trouvé mon chien = passé composé
- il trouva mon chien = passé simple (rare in oral, but still very present in writing)
And it is true that "il trouvait mon chien" would be "he was FINDing my dog", word for word, but that still does not sound right.
could it not mean that someone had asked him to find their dog and he was in the act of looking for it because that is what that last sentence would mean in english
, or would you have to use chercher to get the same meaning, and as quibo said trouvait is more like "founding" which is gibberish