system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon] Why is there a disproportionate amount of electric mouse pokemon?

It seems like every generation has an an electric mouse pokemon. But no water mouse pokemon. No sewer mouse pokemon. The closest thing we have is rattata and raticate, but they’re more like field mice/rats.

Is there a reason why a mouse pokemon would develop with a hankering for electricity?
gpt
The most likely explanation is that most of the electric rodents have a common ancestor that diverged from the Rattata line at some point. The most likely reason that ancestor flourished is that it's electric powers allowed it to survive over its competing prey. 

Picture an early mouse Pokemon. (and it would have to be very early, since its ancestors become so widely distributed worldwide) If it learned even a simple move like nuzzle or thundershock, it would have a massive advantage against flying type predators. (Which, incidentally, there are a lot more varieties of when compared to electric mice) And after a couple dozen generations of refining those electric attacks, biological processes would start to change. The proto-pikachu that best learns to harness electricity would be best suited to survive in this environment. 

TL:DR; Natural selection in a world where it is physically possible for mice to defend themselves with electricity.