Undead were once alive: zombies and skeletons, ghouls, wights, wraiths, vampires and liches. Most can’t be poisoned, many see in the dark, and radiant damage and Turn Undead are the usual answers.
20 undead in the SRD, from CR 1/4 to CR 21; 5 have legendary actions. Average AC 13.4, average 64 hit points.