Dajjal will Born on earth or he is already present from many years and will appear on earth?
Yes. Dajjal is already existing. Dajjal has been arrested and kept in an island and will be exposed closer to Qiyamah as mentioned clearly in the following narrations. Saheeh Muslim Vol 2, page 405; Thakmila Fath hul Mulhim Vol 6, Page414.