alt Hacker NewsAn opus is just short for "magnum opus", and it's a different type of label than a sonnet. A sonnet is a very particular kind of poem, while "opus" basically just means an important work. It can be short or long, and has no format requirements like sonnet (or haiku does).
And fables are not particularly long actually.
To clarify: opus is not the same thing as έπος (épos), which is rendered as epic in English, both as adjective and noun. An έπος is indeed a type of long written work, but opus (work) is unrelated etymologically, they come from different Proto-Indo-European roots.