It is not known to evolve into or from any other Pokémon.
Along with Entei and Suicune, it is one of the Legendary beasts resurrected by Ho-Oh after the burning of the Brass Tower. Of the three Legendary beasts, Raikou is said to represent the lightning strike which ignited the fire that consumed the Brass Tower.
Raikou is a quadrupedal, yellow, tiger-like Pokémon with black stripes and a white underside. It has a thin, light blue tail with sharp angles and a spark-shaped formation at the end. Most of its face consists of thick, white fur and shorter yellow fur around its red eyes. Raikou's muzzle is a light blue "X" and it has a black faceplate with two bumps that cover its forehead, nose, and ears. Raikou has two long fangs that make its appearance similar to the saber-toothed cat. Its large claws each converge to form a single point and never retract. Extra fur hangs off its body in front of its thighs. Raikou has a mane across its back that resembles dull purple rain clouds and allows it to shoot bolts of lighting.
As seen in the anime, Raikou is able to fly by creating storm clouds. It is shown to be distrustful of humans, though it may form some friendship with those who help it. Raikou tends to appear during thunderstorms and when other Electric-type Pokémon are in danger. Raikou is said to represent the speed of lightning and can control storm clouds at will. Raikou races across grasslands while barking cries that sound like thunder. Raikou's cries can also send shock waves in the air while shaking the ground at the same time.
These Thunder Pokémon travel near and far, blasting out electrified whirlwinds. They carry storm clouds on their backs, so they can produce lightning regardless of the local weather; it's even said that they descended to the earth with via lightning bolts. Their extreme quickness lends itself to flight, so they're very hard to capture.
NA:Was that a bolt of lightning flashing by, on the ground instead of the air? Then it could have been Raikou. A rumble of thunder in the distance, but not a cloud in the sky? Sounds like Raikou's roar to us. It is said that this wild Pokémon perished in the Johto region once, but Ho-Oh was so filled with sorrow at the loss that Raikou was resurrected.
PAL:If you see a bolt of lightning flash across the ground instead of from the sky, you may have just caught a glimpse of Raikou. And if you hear a peal of thunder that lasts longer than it should, that may be Raikou's roar. It's said that this wild yet majestic Pokémon once perished in the Johto region, but Ho-Oh, filled with sorrow, resurrected it.
When asked about the basis behind Raikou's design, Muneo Saitō described the process to be difficult, as "Raikou took many twists and turns; there was even a blue thunder version with a drum on its back", but it eventually fell into a tiger-like design.
Its long upper fangs bear a resemblance to those of smilodon, commonly known as the saber-toothed tiger.
Raikou may have also been inspired by thunder and lightning gods of most cultures surrounding the world, mostly the 雷獣 raijū (lit. thunder beast), a thunder demon of Japanese lore and is said to be Raijin's companion. It is said to be a creature that could take the form of various animals.
The flowing purple cloud on its back seems to be based on a cumulonimbus cloud, which is most often associated with thunderstorms.
Raikou could be taken to mean 雷光 raikou (lightning), which is also the inspiration for the name of raijū, or it may be a combination of 雷 rai (thunder) and 皇 kō (emperor). It may also derive from 雷公 raikō (thunder duke), the on'yomi reading of Lei Gong, a Taoist/Daoist deity.