It is not known to evolve into or from any other Pokémon. Reshiram is the version mascot of Pokémon Black, appearing on the boxart. Along with Zekrom and Kyurem, it is a member of the Tao trio, which exists in the legends of the Unova region. Kyurem will become White Kyurem upon using the DNA Splicers to fuse with Reshiram.
Reshiram is a white-colored, bipedal Pokémon of seemingly draconian and mammalian basis, with some of the avian features of feathers. Reshiram's snout is mammalian in appearance, most resembling something vulpine or even canine, or possibly even a goat. Streaming out from the upper side of the snout and outward from the head is a long, voluminous wispy mane, the top of which forms a spike or small head-crest of some sort. Reshiram's face is fringed with spiky features, with one small, pointed extension of it below the chin.
Reshiram's neck is long and slender, with a collar-esque protrusion of fur or feathers seeming held in place by two somewhat glass-like neck-bands, with similar bands present on the "wrists" of Reshiram's forelimbs. Reshiram's forelimbs are expansive, wing-like appendages or some unusual sort of wings themselves; the beginning portions of the limbs are slender and generally conventionally arm-like, but flare out into a more wing-like, feathered section farther on the limb, sporting four claws on the leading edge. On Reshiram's chest is a feathered feature, with the lower portion protruding outward to a point. Long feathers spread out from Reshiram's thighs, its feet with large claws - three in front, and the one in the back positioned at an angle somewhat like high heels. Reshiram's tail is a thick mass of a center-most plume and ribbon-like secondary extensions, surrounded by two large bands. This tail is based on an electric turbine.
In a Unova myth about the twin heroes, as Reshiram sided with the older brother who sought truth, it firmly believes in truth. Reshiram is a genderless species.
Reshiram has a jet engine-like tail which it uses to accelerate itself when flying in high speeds. It is able to send flames from its tail into the air to incinerate everything around it. It can move the atmosphere and change the world's weather by flaring its tail. Reshiram is the only Pokémon that can learn Fusion Flare and Blue Flare, though Victini can have both of them from several events, and Kyurem can learn the former after merging with Reshiram. Also, If Reshiram uses Fusion Flare immediately after being hit by Fusion Bolt, its tail turbine activates and power is doubled. [Source: bulbapedia]