Who are they?
A community that was wiped out by Dhul-Qurnain. His story is narrated in Surah Kahf. These are extremely evil people who will once again resurface. They are not aliens or special creatures. There are people like you and me who were wiped out and Allah says this will be the only community that was wiped out to ever come back again.
21:95 “It is forbidden for any community we had annihilated to return.”
21:96 “Until when Gog and Magog are let loose, and they hasten out of every mound”.
21:97 “That is when the inevitable prophecy will come to pass, and the disbelievers will stare in honor: “woe to us; we have been oblivious. Indeed, we have been wicked.”