The Ultra Ruin.
After a while studying it's lore and the sentences of the only remaining person in that world, here is the conclusion I reached.
That world was, unlike what most people believe, not destroyed by Guzzlord but rather, by humanity itself. Guzzlord was a natural inhabitant of that world, and one that ate toxic waste and rubbish. So, for it to be born, or rather, for Guzzlord to be the only thing to exist...
The Ultra Ruin is set on a massive crumbled building, so massive it covers a surface equivalent to the entire Hau'Oli city. In other words, the entire landmass of the Pokémon World has been turned into buildings, and polluted by mankind to the point where the colour blue no longer exists neither in the sea nor in the sky. To top it off, rather than take responsibility, as humans usually do, they have simply decided to abandon that planet and set off for another place, as it's so contaminated you need massive machines to filter water and the only sustenance available is canned food.
Think about that. The entire world, polluted to an extreme where the only choice of wear is a suit, and in which Guzzlord is the only Pokémon that can thrive, and even the Guzzlord population is diminishing.
This is a future not unlike ours. This is not the first aesop Game Freak has made with the dangers of the progress of technology and the harm to nature it creates: first it was Grimer and Koffing. Then, Trubbish and Garbodor. Now, we literally are faced with a bad future where mankind has simply left the planet and taken with them everything, where the process will most likely be repeated all over again, leaving behind what once was our pale blue dot, and is now blue no more.
