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Beaucephalis is the Dragon of Legend, distantly related to the first Dragon of Legend, who stopped the first dragon-human war. He's a gold Star-class dragon, so he has pure gold draconium in his bones.
When he became of an age when he could start racing (16), it also became necessary for him to choose a human who would be the Dragon Booster. Several advanced racers tried to ride him, but he rejected every one of them - Artha, not quite getting the chance to be tested when Beau decided he wanted Lance's chocolate, and Artha fell off but wasn't rejected. But, that was about to change when the Dragon Eye crew tried to steal Beau. Beau chose Artha while rescuing him from Artha's burning home and Artha was able to become the Dragon Booster.
Beau disguises himself as a red and blue dragon to protect himself from Word and anyone else who wants the Dragon of Legend for a pet, although it seems that he can change to other colours as well, as shown by the episode 'Framed!'. He goes along with what Artha says, most of the time, and it has gotten the both of them into trouble, but he also has a great sense of humor. Now, while that's great, it has embarrassed Artha several times, but all is well as Beau's and Artha's friendship becomes greater as the series progresses. Toward the end, their bond is very close. Beau shows many habilities in the show, like spreading wings to glide, he can absorb other draconium types to use them, mag-shield, fuse with the Dragon Booster as one, and he can spread large claws to climb (it also changes his skin design). He's the strongest dragon as we can see in some episodes, he can overcome wraith gear, bonemarks, and even black draconium, thanks to his bond with Artha.