These are souped-up versions of standard monsters with new movesets and higher damage. Fighting them requires specific "tickets" obtained from other quests, adding a layer of progression to the endgame.
(via 240fps captures):
Q: How do I redeem NPromo items? A: NPromo items can be redeemed at the in-game "Item Box" or through the "Nintendo eShop." monsterhuntergenerationsultimatenspromslab