Zekrom is a black, bipedal Pokémon resembling a dragon or dinosaur, with dark patches on various portions of its body. It has red scleras, white irises, and black pupils, as well as a small horn-like protrusion on the tip of its snout. Its head sports a thundercloud-like crest tipped with light-blue coloration. There are six pairs of pointed teeth visible when its mouth is open, two in the upper jaw and four in the lower. At the base of its neck is a black, stud-like feature, and atop its shoulders are similar, larger features resembling armor padding. The bases of its wings also have such padding, and a fanned, splayed, seemingly webbed shape with a small "wing-spike" on one edge. The lower length of Zekrom's arms extenuate into splayed, seemingly webbed, somewhat baseball-mitt-like formations, with three-clawed, dark-colored hands on the undersides. The front portion of Zekrom's waist is black-colored, with its belly and thighs having thin striations down their length. Its knees are black, as are the foremost parts of its three-clawed feet and spiked heels. When its emotions surge, it enters Overdrive mode and its tail lights up with a blue light.
Zekrom's tail consists of large, round, conical and spiked formation, likened to an electricity generator, with a black inner portion and bands on the tip of the outside portion. This tail produces energy for its attacks and emits an anti-gravity field, allowing Zekrom to fly through the air with minimal effects from aerodynamic resistance and gravity. It can cause several thunderstorms in one area, and it normally hides in thunderclouds. In a Unova myth about the twin heroes, Zekrom sided with the younger brother who sought for ideals. It is a genderless species. It is the only Pokémon that can learn Bolt Strike, though it is commonly associated with the move Fusion Bolt.
Zekrom debuted in a cameo in the opening sequence of In The Shadow of Zekrom!. It made its full appearance later on, where it appeared above the skies of Nuvema Town. Its huge lightning bolt zapped Pikachu, making it unable to use its electricity. It reappeared in Enter Iris and Axew!, where Pikachu recovered its Electric powers after being zapped again by another of Zekrom's lightning bolts.
Zekrom appears on the Unova Pokémon League stage, where it will cover itself in a ball of lightning and hit the stage. Any fighters it touches will be damaged and if it hits the side of the stage, the stage will be tipped. It will also destroy the steps attached to the stage, if there are any.
Zekrom's jet-black body contrasts sharply with the pure white of Reshiram, but the two do have some similarities: both are Legendary Pokémon said to have world- destroying powers. Zekrom's tail acts as a generator that can scorch the world with lightning, but it is said to assist those who wish to build an ideal world.
This Pokémon was unavailable prior to Generation V.
Zekrom was first revealed simultaneously alongside Reshiram on both the official English and Japanese Pokémon websites on May 28, 2010. This makes them the first two Pokémon simultaneously revealed worldwide.
On May 31, 2010, two weeks prior to the official revelation of Zekrom's type, Shōko Nakagawa spoke about her love of Electric-type Pokémon on her blog and mentioned that because of that, she wanted to get a Zekrom. This statement was then quickly removed.
In the Generation V games, Zekrom, Reshiram and Kyurem use different variations of the same battle theme. In Pokémon Omega Ruby and Alpha Sapphire, however, Zekrom's version (featuring electric sound effects) is used for all three Pokémon.
Zekrom and its counterpart Reshiram are based on the concept of yin and yang in Taoism, directly referenced by Zekrom's Japanese species: the Black Yin Pokémon. According to Ken Sugimori, Zekrom was designed to have a tougher and more masculine design to contrast to Reshiram's light, airy, feminine design. While the black design matches Zekrom's yin species names (yin being characterized by dark), its masculine design contrasts with yin (yin being characterized as feminine). Zekrom's tail, the center point of its design, is based on an electric turbine. Appearance-wise, it somewhat resembles a midsized to large theropod dinosaur, such as Allosaurus or Tyrannosaurus, and a European dragon. The crest on its head is based on a thundercloud. The mythology about how Zekrom and Reshiram came about, regarding two brothers infighting, also bears some similarities to Romulus and Remus or Cain and Abel. There is also some resemblance to the Zoroastrian belief of two equally powerful deities, one representing light and truth, and the other representing lies and darkness. In this case, Zekrom can be taken to resemble Angra Mainyu, as opposed to Reshiram resembling Ahura Mazda, although Zekrom is not evil; rather it is an equally powerful opposing Legendary Pokémon.
Zekrom is derived from 黒 kuro (black). Ken Sugimori has also stated that Ze was used because it was a harder, stronger sound than Reshiram's Re, and Zekrom is a "powerful, strong character."