Jephthah implied that he was willing to go as far as bringing a human sacrifice as a burnt offering in case of victory. Obviously, Jephthah did not expect an animal to be the first to come out of his house and meet him at his homecoming from battle. He knew it would be a human. It could be his wife, his daughter, or another person in his household, such as a servant.
31 then whatever comes out from the doors of my house to meet me when I return in peace from the Ammonites shall be the LORD’s, and I will offer it up for a burnt offering.”