Recently, some people discovered that Shokotan, a presenter of Pokémon Smash has hinted on her blog that Keldeo will have a new form, called the Resolution Form. Although no pictures were provided, this was also foreshadowed in the previous CoroCoro issue, in which Keldeo was said to be engulfed in mysterious light.

Pictures of the new form are expected to be shown in the next CoroCoro issue.

Source: Serebii

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I have to admit that I'm pretty mad about this.

They've given way to many new formes to the legendaries in this generation.

SO MANY FORMES. SO LITTLE TIME.

...No, honestly. We have too many forms. It's getting kinda silly, really.

Keldeo's Resolution Form was revealed on the cover page of CoroCoro. Keldeo is shown with a blue horn on its forehead, a longer mane, and 3 differently colored crests above its horn (green, yellow, and dark blue). The three crests appear to represent the colors of the other 3 musketeer pokemon: Cobalion (dark blue), Virizion (green), and Terrakion (yellow).

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Kyurem's new form consisted of either Zekrom's or Reshiram's features fusing with Kyurem's. It seems that they are using the same logic with Keldeo, if the three extensions actually resemble the other three of the group.