For sure: 1) Never use 'fressen' for humans unless you want to insult them and 2) it is never wrong to use 'fressen' for animals. Sometimes it's ok to use 'essen' for animals, too. It largely depends on the circumstances and on the animal in question. In general, as a rule of thumb: the cuter the animal and the more affection you feel for it, the more likely it is for people to use 'essen' instead of 'fressen'. But it really is save to stick to 'fressen', just accept that 'essen' sometimes is acceptable, too. (For cats we indeed mostly use 'fressen')
I don't think it's that simple. Most people use 'fressen' for their dogs. For a hamster 'essen' is used a lot. There isn't any reliable logic behind it. I guess for non-native speakers it's best to just stick to 'essen' for humans and 'fressen' for animals. Just be aware that you can use 'essen' for animals too.
The comments about "essen" vs. "fressen" are... Well, friendly, if not super helpful, since there is apparently no rule or logic to it.
I don't think I've seen "fressen" used anywhere in Duolingo yet. Maybe that's to avoid confusion for students who are learning, but isn't something "Das Pferd isst" pretty clearly wrong?
It does, because that would be saying "THE fish", implying that it is a specific fish. It's like saying "the dog eats hamburgers" vs "the dog eats the hamburger"