#relict

Each time a crossover has occured in Monster Hunter, one monster in particular was associated with the new threat. When Behemoth was encroaching on the New World, a shard of aetheryte empowered a humble Kulu-Ya-Ku, turning it into a surprisingly powerful opponent, protected from status and traps. In the end, it had to be slain to break the aetheryte's mutation and help the Moogle.

To a lesser extent, we caught a glimpse of Dodogama... well, right when it was crushed and broken by Behemoth itself

Next, when a Leshen was attempting to take over the Ancient Forest, there is notably a few Pukei-Pukei mercilessly targeted and killed by the malicious relict. One lucky individual escaped death and was freed by Geralt from the constrictinv vines. This makes a charming show when that Pukei, though still injured, later joined the Witcher in attacking and ultimately slaying the Leshen. For helping its bird wyvern friend, one of the local Felynes offered a decoration to Geralt, who kindly instructed it to gift the reward to Astera's top hunter.

Sadly, there is no such help against the more powerful Ancient Leshen, though it certainly proved the humble Jagras and Revultures could be terrifying threats if properly directed. And the most recent crossover, revolving around an Omega frame invading the Forbidden Lands, introduced a unique form of Nerscylla. This clone, created by Omega Planetes, is one of the highest ranked base monsters in the game, R7 alongside Arkveld itself, and serves as a dangerous foe. The principle purpose of the clone was to scout the area and eliminate / identify potential threats.

A second Nerscylla Clone was seen in the battle against Omega Planetes, serving as a defensive unit to attack the hunter while Omega Planetes prepared its devastating Delta Attack. Ironically, the Clone was the means to survive this ultimate blast, as slaying it created a forcefield strong enough to withstand Delta Attack and prevent Omega Planetes enhancing its power.

Of course, the crux of Omega Planetes's motivation was to find powerful monsters to defeat and assimilate. In the Forbidden Lands, it identified Jin Dahaad as the most powerful monster within its area of operation and personally set out to slay the leviathan. Omega Planetes overpowered Jin Dahaad decisively, though was prevented from killing it and harvesting its powers by the hunter. An alternative scenario was offered where Omega Planetes had successfully assimilated Jin Dahaad's date, resulting in a Savage Omega form equipped with icy abilities.