#020: Raticate
Evolution is an unpredictable and poorly understood process, but certain patterns can be observed. Evolved pokemon tend to be bigger and stronger, with larger claws and jaws than their predecessors. Often the cute, somewhat hapless look of a pokemon fades away as a fuzzy pre-evolution becomes a fearsome beast. This is why the semi-precious stone known as the Everstone is so valued: many trainers prefer their companions to stay small and endearing, to say nothing of the feeding and housing concerns that come with keeping large pokemon.
It is not for this reason that Everstones are often given to Rattata or placed in Rattata nests, however. After all, only a niche few find Rattata cute in the first place. Instead, this is a common practice because the unassuming Rattata, dangerous only in large swarms, evolves into one of the biggest menaces to civilisation in the whole of the Kanto region. Combine teeth that can munch through concrete or even steel beams with a penchant for gnawing and you have a recipe for urban disaster.
In truth, Raticate are not vindictive; more-so than humans, they are engaged in a pitched battle against their own terrifying chompers. Their unusually powerful teeth, designed to unleash Hyper Fang attacks capable of turning the strongest would-be predators into months of food for their young, grow constantly. If not worn away regularly, they will eventually prevent the pokemon from eating and, in the worst cases, pierce the roof of the mouth, damaging the brain. The process only speeds up with age, and sadly, few Raticate die of natural causes.