The reason I chose the name "PikaThunder" is because my favourite pokémon is Pikachu, an electric type which can deal thunder attacks.
The reason I like Pikachu is not just because it's "cute". It is because it's attacks can be quite powerful, and makes a good Pokémon to have if you train it. It's attacks also look pretty cool. I tend to not want to use a thunder stone on Pikachu simply because after I do he can't learn anything any more unless I teach him a HM/TM or by Tutoring. But, I did it anyway. I honestly only did so to raise its power.
I also like Eevee because of its various Eevolutions. My favourite one has to be Umbreon, due to its dark look. I've always liked Pokémon with a somewhat dark look, makes them seem powerful and surprising.