in zuldrak there is the quest Disclosure - Quest - World of Warcraft
the npc that escorts u around u can keep and he is always agressive to other mobs but they do not attack him back. the way i kept him was as soon as i got to the upper level and clicked the coffin i took the teleporter just as the quests went 1/1 complete. from then on i was able to keep malmortis. then just mount up and fly away. when u land and demount he will spawn and attack any hostile mobs in sight. if there is no hostile mobs he will just stand there and say "follow me". to move him to other locations just mount and fly away.
me and my mate chilling in durotar
He also goes on zepps
Edit: he also survives logging out and back in