It was once commonly believed that the world would end not because of solar expansion, but with a Swalot growing so large that it swallowed the world whole. Swalot do not grow all that quickly, and their size at evolution is small, so few people think of them as large pokemon. Yet they never stop growing as long as they live, and although their lives are often ended by accident, hunger, or violence, none have ever been confirmed to die from old age; many scientists wonder if these pokemon age at all.
History obviously contains no record of a Swalot devouring the world, for such records and any would-be readers would be devoured as well, but it does offer an explanation of how that idea developed. Hoenn's landscape has long been shifting from the warfare of the gods, and the names of settlements in its antiquity are numerous and ever-changing, so it is difficult to separate fact from fiction regarding lost cities. Yet it certainly appears that a few, such as Bigroot Town, were indeed eaten by hungry Swalot. Many other cities record desperate military campaigns against a single one of these oversized poison pokemon, including gruesome accounts of soldiers grabbed with its giant whiskers and swallowed whole before winning a bitter victory.
Oftentimes, enemy settlements would put local wars on hold to lend each other aid against Swalot, for they feared that if they did not unite, the Swalot would grow too big for the surviving city to confront alone. They did so because of their own belief in the story of a Swalot apocalypse, and it was through this belief that a true Swalot apocalypse was avoided, for they did so until the day came when technology grew so powerful that no pokemon could swallow the world.