Ýes exactly. Animals "fressen", while humans "essen". It really seems though as if duolingo should specify this somewhere, if it hasn't been done so yet. There have been a ton of questions about this lately. In colloquial speech, many native speakers do use "essen" for "fressen" (I know I do, though you shouldn't do it the other way around and use "fressen" when talking about humans, unless you want to be rude), but it's technically not correct. It's just one of those little quirks that each language has, like the two different "to be" words in Spanish, or the different words for meat on a living animal (pig, cow, calf, sheep...) vs. a dead one (pork, beef, veal, mutton...) in English.
Duolingo has specified it somewhere, when you scroll down on animals it shows how to conjugate it:
Unlike English, German has two similar but different verbs for to eat: essen and fressen. The latter is the standard way of expressing that an animal is eating something. Be careful not to use fressen to refer to humans – this would be a serious insult. Assuming you care about politeness, we will not accept your solutions if you use fressen with human subjects.
The most common way to express that a human being is eating something is the verb essen. It is not wrong to use it for animals as well, so we will accept both solutions. But we strongly recommend you accustom yourself to the distinction between essen and fressen.
Fortunately, both verbs are conjugated very similarly:
essen = fressen (for animals)
ich esse = ich fresse
du isst = du frisst
er/sie/es isst = er/sie/es frisst
wir essen = wir fressen
ihr esst = ihr fresst
sie/Sie essen = sie/Sie fressen
In general: never.
Think of frisst as "feeding"--in the sense of what animals do to ingest food. Essen is "eating"--in the sense of how people consume food.
But as far as the present tense goes, "eats" and "is eating" (or "feeds" and "is feeding") are essentially equivalent. So "Die Kuh frisst" means both "the cow feeds" and "the cow is feeding". There's really no difference.