Final answer:
The book opens on the eve of Harry's thirteenth birthday, and Hagrid introduces Hippogriffs in class. Lupin prevented Harry from facing a Boggart due to fears it might take the form of Voldemort.
Step-by-step explanation:
Harry Potter and the Prisoner of Azkaban opens on the night before Harry's thirteenth birthday. In the Care of Magical Creatures class, Hagrid introduces the students to Hippogriffs, which are large, dignified creatures that are a cross between horses and eagles.
As for the incomplete part of your question, Professor Lupin did not allow Harry to face the Boggart in class because he was concerned that it would take the shape of Voldemort, which might have caused panic or distress among the students.