While it is not known to evolve into or from any Pokémon, Keldeo has a second form from Pokémon Black 2 and White 2 onward, which requires it to know the move Secret Sword. Its original form, Ordinary Form, will then become Resolute Form. Prior to Generation VIII, Keldeo will change into Resolute Form if it learns Secret Sword, and will permanently remain in that form as long as it knows the move; in Generation VIII, Keldeo will only temporarily change into Resolute Form if it enters a battle knowing Secret Sword.
Keldeo was leaked on a photo of a page from CoroCoro magazine on February 13, 2012, and it was officially revealed on February 15, 2012. Its Resolute Form was officially revealed June 13, 2012.
Keldeo is a quadruped, colt-like Pokémon with a cream-colored body. It has a short, rounded snout with large nostrils, and blue eyes. It has a large, feathery red mane, long, dark blue, ridged eyebrow-like protrusions, and a single cream-colored horn that curves slightly. The back of its head, as well as its neck, is covered with fluffy, light blue fur. Keldeo has blue hooves, which are capable of ejecting water. Its tail is light blue with white spots and resembles an elaborate feather in a cap.
Upon learning Secret Sword, Keldeo transforms into its Resolute Form. The dark blue protrusions become shorter and lighter in color, resembling ears. Its horn becomes dark blue, and grows larger and ridged. On the side of its head are three feathers colored aqua blue, green and orange.
Keldeo was trained by the other members of the Swords of Justice. Keldeo tends to cross the world, and often appears at beautiful watersides. Keldeo also travels in order to further improve itself through training. It can walk across water by ejecting water from its hooves. It is said Keldeo must endure harsh battles in order for the forehead horn to develop, which then its true power will be awakened.
It is thought that when Keldeo becomes resolute, its body fills with power and it becomes swifter. The horn on its forehead is filled with the hidden power. The horn can change into a sword which can cut through anything.
Long ago, a fire started in the Moor of Icirrus because of a war among people. In this fire, Keldeo was separated from its parents, but together, Cobalion, Terrakion, and Virizion took care of Keldeo. They acted as its parents, and they taught it knowledge and moves for survival. Keldeo eventually grew and even surpassed the three Pokémon. One day, Keldeo took off from the forest with no one knowing why.
When the four meet again, they teach Keldeo the move Secret Sword.
Keldeo debuted at the Pledge Grove, where it waited for Cobalion, Terrakion, and Virizion to arrive and teach it Sacred Sword. After acquiring the move, Keldeo follows its mentors to where they would confront the Shadow Triad who tried to capture the trio earlier, which was near the P2 Laboratory. There, the Shadow Triad are locked in combat with Cilan, Chili and Cress, the Striaton Gym Leaders. It first appeared in The Shadow Triad.
Resolute Form Keldeo appears as a Poké Ball summon in both the 3DS and Wii U versions of the game. It slashes opponents with Secret Sword.
NA: This Water/Fighting-type Pokémon is a lot like a multicolored unicorn. In its Resolute Form, its horn will grow longer and its vision is keener. It is this Form that appears on the battlefields of Smash Bros. Be wary of its Secret Sword attack—Keldeo's charge will send anyone hit flying.
PAL: The Mythical Pokémon Keldeo looks a lot like a multicoloured unicorn. In its Resolute form, its horn grows longer and its vision improves. It can also use Secret Sword in that Form, and that's what it will do in this game. If Keldeo starts charging after you, watch out - that huge horn will send you flying!
Keldeo returns as a Pokémon summoned from the Poké Ball, retaining its behavior from the previous game.
Pokémon Mystery Dungeon: Gates to Infinity: Keldeo is an important character in the storyline where he was a friend of Virizion, and a Magnagate savant. He was imprisoned in Glacier Palace, and there he wrote a letter to Virizion telling her that they weren't friends anymore. This letter caused Virizion to stop believing in friends. After being rescued by Umbreon and Espeon, he apologizes to Virizion for writing the letter. He joins the player's team after the player Pokémon returns to the human world..
This Pokémon was unavailable prior to Generation V.
Keldeo's Resolute Form in Generation VIII is the only instance of a form becoming a temporary battle-only transformation despite previously being an indefinite transformation that occurred on both the field and in battle.
Keldeo is most likely based on the legend of the kelpie, a supernatural water horse that sometimes appeared as a lost pony. Keldeo is also likely based on the kirin, also referred to as the "Chinese unicorn" which are often depicted with a single backwards curving horn and possess the ability to walk on water. Keldeo also draws inspiration from the character of d'Artagnan from the novel The Three Musketeers. Like d'Artagnan, it is the latest and youngest addition to the group. Keldeo's tail also resembles d'Artagnan's trademark feathered cap.
Keldeo may be derived from kelda (Old Norse for spring/fountain) or may be a combination of kelpie (a water horse from Celtic folklore) and rodeo (a sport involving skills with horses, cows, and other livestock). It may also involve deo (Latin for god).