Answer: Hoarse
This pun works because of the fact that the words "horse" and "hoarse" are homophones. Homophones are words that have the same sound, but different meanings. In this case, the sentence states that the pony could not talk because he was slightly hoarse. However, if the word were "horse," the statement would indicate that the pony was a little horse (in terms of size), which is a true statement.