Answer:
The huge rushing shadow Zebra saw was a car. A year ago Zebra was crashed into a car. In the text it states, "a year ago, racing down Franklin Avenue, he had given himself that push and had begun to turn into an eagle, when a huge rushing shadow appeared." I believe that shadow to be a car because what else is huge that would be on the streets. After crashing into a car when he awoke the doctors told him "he would never be able to run like that again."