Complex sentence: Horace listed Eupolis, Cratinus, and Aristophanes (in that order) as the most prominent writers of Old Comedy, noting how they would "single out" the immoral in their comedies.
Split into simpler sentences: Horace listed Eupolis, Cratinus and Aristophanes (in that order) as the most prominent writers of Old Comedy. Noting how they would "single out" the immoral in their comedies.