It is frisst because esst is the incorrect form for the sentence. I think you mean 'isst.'
Fressen is for animals, or to eat like an animal; essen is for people or animals but is more polite to use for people. (It's rather disrespectful to say "ein Mann frisst." But, if his manners are very sloppy or he's basically eating like an animal, it could be used.)
Spaced repetition. Duo repeats words in sentences that are grammatically correct, but not necessarily logical.
The various language trees contain sentences in which animals and insects eat bread, rice, sandwiches, vegetables and strawberries; drink beer and milk; and read newspapers and books.
(Apparently, horses love apples, but they can choke on the core. It's safest to feed them apple slices.)
I do as well as listen to lessons and podcasts for about 5 hours a day at work. So its not easy to remember every detail after only 17 days. Plus Im really busy so its easier to just let one simple answer come to me rather than stop everything Im doing and go searching for it. Thanks Ill take a look at my accusative notes later.
einen = accusative case because 'Apfel' is a direct object here (not an indirect object)
I suppose you could say something like 'Es frisst mir einen Apfel', where' mir' would be an indirect object, but that wouldn't make much sense because what possible benefit or effect would I gain from the horse eating an apple?