I need help writing an essay on Horace's "Ars Poetica" in light of Plato's and Aristotle's arguments on literature.
The first issue you might want to discuss in your essay is the question of influence. Although the Aristotelian corpus was available in the edition of Tyrannio the Grammarian and Andronicus of Rhodes, Aristotle's works were rarely referenced in writers of Horace's period and that mainly from florilegia; the major commentaries on his work began to proliferate a few centuries later. Philodemus was apparently aware of Aristotle's (now lost) treatise On Poets, but we have such limited knowledge of that work that it is unlikely to be of much use in your essay.
Plato was considerably better known, with his work being widely admired for its prose style as well as its philosophical contents. Among Middle Platonists, Ion was rarely studied and Phaedrus a central part of the curriculum; the Republic was a more advanced element of the standard Platonic curriculum. Horace's discussion of the inappropriateness of yoking a human head to a horse's neck and avian plumage may be influenced by the Platonic demand for organic unity at the end of Phaedrus; the reference to Socratic dialogues seems most likely to be to the final section of Phaedrus as well.
Horace differs from both Plato and Aristotle in that he is a poet writing in verse a sort of practical manual for poets, whereas the works of Plato and Aristotle were more philosophical. Two areas that might be especially relevant to your essay are the way both philosophers criticize excessively difficult or flowery language that obscures meaning, something we also find present in Horace. Although Horace does not set forth an elaborate analysis of the moral effects of poetry, in the final section of the Ars Poetica, he does emphasize that poets derive some of their interest and authority from advocating traditional morality.
The final issue you should discuss in your essay is Horace's strong conception of the nature of genre and how writing style must conform to specific generic demands. This is quite similar to discussions we find in Aristotle's Poetics.