What happened to the bull isn't the most important information; the author wants to know why his son didn't come home. Saying "the bull is dead" kind of answers that question, but only via inference ("oh, the bull must have been killed by his car"), which is why the father was initially confused.
And then "the" should be "a" because the bull hasn't been introduced yet.
> "I'm OK, my car hit a bull."
What happened to the bull isn't the most important information; the author wants to know why his son didn't come home. Saying "the bull is dead" kind of answers that question, but only via inference ("oh, the bull must have been killed by his car"), which is why the father was initially confused.
And then "the" should be "a" because the bull hasn't been introduced yet.