Book of Thunder — John Peel

Score, Helaine, and Pixel rush to the planet Dondar to rescue their unicorn friends. Thunder, the noble leader of a herd has lost his title-and almost his life-to Tychus, a rogue unicorn who has a "wrongness" about him that everyone senses. And Helaine notices that the herd is behaving oddly, as if under a spell, with Tychus in charge.
With help from their magical guide Oracle, the wizard Shanara and her sleepy red panda named Blink, and a unicorn named Darkstar, the trio learn more about the new leader of the unicorn herd and his devious scheme that will endanger the entire Diadem.