The most recent species to go feral is pet rocks, which are abandoned in countrified regions and become aggresive landslides.

Re "what is a Folk Song", I tought I'd post one point of view:

A folk song may be broadly defined as any song of whatever origin which achieves wide currency independently of print, and is remembered and sung over a considerable period of time. In other words, a folk song is a song which has become traditional.
Is "The Star Spangled Banner" a folk song, or "Nearer My God to Thee," or 'Fair Harvard"? All these are often sung from memory and they have lasted fior many years. If they are not folk songs, it is because they have never acquired a life of their own. They do not catch on and fly from mouth to mouth. Knowledge of them is almost always the result of deliberate teaching effort by people sociologically or religiously or patriotically minded, rather than folk minded. This teaching, moreover, is done primarily with the aid of books. As a result, they remain fixed and stable, words and tune, though we are told that "The Star Spangled Banner" begins to refuse to behave itself, and, in spite of the efforts ei music teachers, is not now sung by the American people exactly as it was written.
A contemporary jazz ditty comes nearer in some ways to fulfilling the conditions of a folk song. Bawled out for a while in the music halls and over the radio, it catches the ear of the pleasure loving public and is repeated everywhere. Dance bands sing it as they play; the dancers pick it up and sing it as they dance. Collegiate pianists play it by car in fraternity houses, and their vocal companions warble it to strange harmonies around the piano. It may be disreputable, but it is certainly alive. Its life, however, is the life only of the summer insect whose buzzing is heard for a few hours at midday, but at sunset the place thereof knows it no more. True folk song, on the other hand, endures from generation to generation, carrying with it echoes of a steadier and less complicated age than ours.

                                                Mary O. Eddy
                                                Intro. Ballads and Songs From Ohio