Then the Spirit of Yahweh came on Jephthah, and he passed over Gilead and Manasseh, and passed over Mizpeh of Gilead, and from Mizpeh of Gilead he passed over to the children of Ammon.
Compare Gideon’s similar response to the gift of God’s Spirit. The masculine gender of the Hebrew word translated "whatever" can apply to a person or an animal, but Jephthah was probably thinking of an animal.