"To unlock Akantor and Ukanlos, clear all high-ranked hunting quest.
To unlock Alatreon, do the same, plus marathon hunting quests.
To unlock Deviljho in Elder's Quests, clear all 4* and 5* quests.
To unlock your nightmare (4 monsters in arena, which is Black Diablos, Deviljho, Ice Agnaktor and Jinouga) clear all Guild 5* hunting quests.
There, you have it."