Buster is a Unicorn at Camp Jupiter.
The Trials of Apollo
The Tyrant's Tomb
Buster is seen in the Stables staring dead eyed at Apollo. His horn shavings are used by Pranjal and Meg McCaffrey to help ease the scratch on Apollo’s gut.
During the second wave of Caligula's Assault on Camp Jupiter, Meg leads weaponised unicorns into the fight, Buster being one of them. He and the other unicorns take on the Zombies where Meg find out that the unicorns are immune to zombie scratches.