The winters of Sinnoh can grow bitterly cold, and Glameow are a thin pokemon whose fur is not coarse enough to keep them warm through the winter. As the legend goes, when the first Glameow evolved into its original new form, it found itself no warmer than before, but a great deal stronger, and resolved to use its strength to conquer the cold. At first, it bullied its way into the nests of other pokemon, much as Purugly do today, but no nest was warm enough, so it grew greedy and stole the blubber from Snorlax in the hope of gaining its warmth.
Some say it stole this blubber while Snorlax slept – no difficult task, given how often and deeply a Snorlax will sleep - and was cursed by Arceus itself for its crime, by being twisted into the ugly shape of modern Purugly. In other versions of the tale, Snorlax was awake, its whiskers were broken in the fight itself, and its body grew misshapen because although it had stolen Snorlax's fat, it was not a naturally thick-bodied pokemon like Walrein or Snorlax itself, and therefore had no easy way to wear what it had stolen.
And yet Glameow to this day strive to evolve, for they value warmth and power far more than they do beauty in human eyes. Purugly may be less graceful than Snorlax, but their bulk similarly protects them from the elements and adds to the power of their body slams – a far more rational explanation for their front-loaded and bulky appearance than ancestral damage or a divine curse. Then again, the pokemon world does not always run on reason, and it remains difficult to understand how a pokemon as vain as Glameow could evolve into such a slovenly beast.