Some pikachu variations!

The top pikachu is a standard, wild-caught Kanto pikachu. This pikachu sports pale yellow fur, simplistic cheek markings, and an average-length tail and ears.

The bottom-left pikachu is one bred for competitive fighting. This pikachu sports "erratic" markings, a darker fur colour (potentially "shiny"), a thicker build, a large jaw, and shorter ears and tail.

The bottom-right pikachu is one bred for contests. This pikachu exhibits soft and flowing fur, a very long tail with an exaggerated shape, long tapered ears, and breed-standard markings (particularly the stripes on its back, which must be well-pronounced and two in number).

Drawn on SAI.