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 quadruped, 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 covers 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 races across grasslands while barking cries that sound like thunder.
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.
Raikou may be particularly based on the 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 a tiger, monkey, blue wolf wrapped in lightning, and many other animals. Raikou is specifically based on its tiger form, while the Pokémon Manectric is based on its wolf form.
In terms of physical build, Raikou may be based on the Smilodon fatalis, a well-known but extinct species of wildcat often referred to as a saber-toothed tiger. Its color and pattern scheme are reminiscent of that of a Bengal tiger. The stripes that decorate Raikou's body also can be compared to zigzagging thunderbolts and perhaps even scars from claws. The white ruff around Raikou's face is akin to the Siberian tiger's thick fur around its neck. In the anime, Raikou is shown to be very agile, swift, stealthy, and a powerful attacker, just like a tiger. The gray "plate" atop its head may be similar to Entei's head ornament, being either a helmet or crown that signifies courage and power.
The flowing purple mane on its back is likely meant to invoke the image of a cumulonimbus cloud, the cloud type 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.