In fact, it was not only the Hippogriffs that were allied with the centaurs. All the creatures that were grazing in the meadow were part of their alliance as well. This was why the Centaurs appeared when William's group arrived in the meadow.
The Wild Bison, Moose, and Deer had already trapped William's group in an encirclement and were only waiting for Bastian's order to attack.
The only reason why they were still being civil was because of the fact that William was radiating a presence that made them not want to have a conflict with him if possible.
"Leave!" Bastian ordered. "This is your last warning. The next time you return, we will treat you as enemies!"
William sighed internally because they were left with no other option but to leave the meadow.
"Okay, we will leave." William nodded his head. He then turned to his group and made a hand gesture. "Let's go."
Cid and Aerith were frowning, but they still followed William's orders.
