The fact that it's so powerful leads me to believe that there is much more to this rat than things may seem. Here's my spin on it and I'm well aware of how over the top ridiculous this story is but I feel like having some fun here; Ash's Pikachu might be the result of some sinister experiment from Team Rocket involving Raikou. They wanted to make pokemon supersoldiers as sort of a test run for Project Mewtwo. To increase Pikachu's power to the same level as Raikou, they used a single hair from the beast, extracted some DNA, and injected a few genes into the pokemon while it was still in the egg. When it hatched as a Pichu, they ran the usual battery of tests to make sure it's body was working correctly and everything went according to plan. When it was due to begin combat training, it eventually escaped from the facility and somehow found it's way to Oaks lab. With the loss of a very important prototype, the supersoldier project was cancelled and Team Rocket's scientists turned all their focus to creating Mewtwo. Now back to Pikachu. Professor Oak kept the Pokemon for many years and eventually, it evolved. When Oak found Ash, he gave Ash this Pikachu for whatever reason and the two started their journey.