Boggart

A Boggart is a shape-shifting creature that takes on the form of the viewer's worst fear. It was created by J.K. Rowling.

It is unknown how some people react to boggarts, and what they see, but in Zeo RyDer's potion's class, in the Polyjuice Potion lesson, he had each student use the potion with his, Krystal's, Miles', or Tiencho's hair.

Those who were Krystal, the boggart turned into Dr. Andross.

Those who were Miles, the boggart was Shadow the Hedgehog.

Those who were Tiencho, The boggart assumed the shape of Po.

When Zeo, and students under the polyjuice of him, the boggart is a Zombified Krystal. One instance, Zeo was so disturbed, he accidentally swallowed his tounge in fear, showing how much he's afraid of Krystal's death.