He struck them from Aroer until you come to Minnith, even twenty cities, and to Abelcheramim, with a very great slaughter. So the children of Ammon were subdued before the children of Israel.
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.