Answer: B) Cow.
Explanation: In the given passage we can see the description of how Paul and Tim were riding Buster with a makeshift saddle. From the information expressed in the excerpt, we can know that Buster is a cow, because in the phrase "It seemed like an hour when, with a loud and angry “Moooo!” he took off quickly" the speaker says that Buster says "mooo" which is the sound cows make.