The vast crystal structures on Pheromosa's home world, whose unusual chemical signatures, together with its extreme radioactivity, alerted astronomers to the location of Pheromosa's home planet, were not built by the Pheromosa. Indeed, they could not have been, for Pheromosa's clawed hands and frail arms do not allow it to even lift the mud bricks and crystals used to construct the Ultra Desert's vast ruins. In fact, at the time when those buildings were functioning homes, fortresses, and monuments, Pheromosa's ancestors were no larger than a Wurmple or Caterpie.
The builders of the Ultra Desert, by and large, were responsible for their own demise. Pheromosa did not cause the bitter feuds which boiled over into apocalyptic warfare, nor did their bodies permit them to construct nuclear weapons, and the radiation that drove many a survivor into an early grave also accelerated the evolution of the Pheromosa. But Pheromosa did act as purifiers, cleansing the Ultra Desert of anything they considered unclean; the last survivors did not resist, only describing them as "nature's rightful revenge".
Humanity has thankfully thrown away their weapons of war, if that is what the Pheromosa so despised. And if it is not, there are still many pokemon in this world who possess the power to stop them. Yet one can still not help but be disturbed at how reluctant those Pheromosa who fall through the Ultra Wormhole are to touch virtually anything on Earth – as if they view this world, too, as horrifically unclean.
May it never be purified!