Replace the question mark with a number

One day the teacher decides to play an animal game. She holds up a picture of a giraffe and asks if anyone knows what it is. No one raises his/her hand. The teacher says "See it's long neck? What animal has a long neck?"
Sally holds up her hand and asks if it is a giraffe.
"Very good Sally," the teacher replies. Next she holds up a picture of a zebra. None of the students holds up his/her hands. "See the stripes on this animal? What animal has stripes?"
Billy holds up his hand and says it is a zebra. "Very good Billy," the teacher replies. Next she holds up a picture of a deer. None of the students recognized the animal.
"See the big antlers on this animal. What animal has horns like this?"
Still no one guesses. "Let me give you another hint, it's something your mother calls your father."
Johnny shouts out "I know what it is, it's a horny bastard."