"do cachorro" can be possessive, too, right? And just the more plausible context tells us that it is "saves the cat from the dog" and not "saves the dog's cat"? Is there another way to say one of these so that it can be less ambiguous when needed? The dog as cat-owner part may be ridiculous, but what about, e.g., "She saves the woman's cat" vs. "She saves the cat from the woman"? "Ela salva o gato da mulher."
There is the context that a dog possessing a cat isn't as plausible as a cat needing saving from a dog. Not perfect context, and of course sometimes there are silly implausible things to translate in Duolingo, but I prefer when we're informed of more plausible translations. Even if that means marking wrong a technically possibly accurate but less plausible translation.