The correct answer is: "its"
Explanation:
The textbook definition defines noun as a person, place or a thing. Everything that does not belong to a person category assumes to in a "thing" category (if not places). The pronoun we use for anything that is not a person is "it." Here in this case, "one of the horses" phrase is used. A horse does not lie in the person's category. Therefore, we will use "it." In the case of possessive pronoun, the possessive form of "it" is "its." Therefore, the correct answer in this case is "its."
