While it is not known to evolve into or from any Pokémon, it has a second form, which is activated by triggering an event in the Pledge Grove, where Cobalion, Virizion, and Terrakion teach it Secret Sword, and it changes its form permanently as long as it knows Secret Sword. Its original form, Ordinary Form, will then become Resolute Form.
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 pony-like, with a large, feathery red mane, long, dark blue, ridged eyebrow-like features, and a single cream-colored horn that is slightly curved. The back of its head, as well as its neck, has light blue-and-white-spotted coloration with spiky edges at the lower portion. Its body is cream-colored, with blue hooves. Its blue-and-white-spotted, lifted and feathery tail is comparable to an elaborate feather in a cap.
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 three meet again, they teach Keldeo the move Secret Sword.