The kings of Alola imported Yungoos to combat the Alolan Rattata, an invasive species which had arrived with humans on the islands and drove many of its bird pokemon to extinction. Yungoos, however, found the Rattata (unlike those in their natural habitat) to be far too large and menacing to eat; when they did fight, it was over access to the very food pokemon Rattata were eating.
Worse, Yungoos devoured some species which Rattata instinctively refused to attack. Alolan Ekans was no Seviper – they were a rare species, known for its relatively small size and weak venom, and Yungoos played the role of their Zangoose cousins in Hoenn until they annihilated the insular subspecies entirely. Ekans were not the only snake pokemon on the archipelago, and the region's population of Dratini would be remembered in Alola's folklore as Yungoos' bitter rival – a snake they could never catch, for they shed their skin whenever a Yungoos bit down on them. In truth, Dratini do not always make such a clean escape; scientists have noted a decline in the population in Alola, both from outright predation and because Dragonite have nested less often in Alola than before Yungoos' introduction, for fear of their slow-growing children's survival.
Yungoos have become such a blight on Alola's ecosystem that a few maverick scientists propose introducing even larger terrestrial predators, such as Luxray, to extirpate them and Rattata alike – but the public resoundingly reject these proposals, determined not to make the same mistake again. Yungoos control efforts have typically relied on mass capture and strict training; earlier attempts at poison and hunting, like the introduction of the Yungoos themselves, did more harm than good.
The world of pokemon, after all, is a very difficult thing for humanity to predict - let alone to control!
