The Ammonites appeared in the Ehud story as allies of Eglon, king of Moab (Judges 3:13). The reference to them at this point rather than to the Moabites is almost certainly because of their significance in the current situation: Israel is oppressed by the Ammonites again because they have been unfaithful again: they never learn(!).1
11 And the LORD said to the people of Israel, “Did I not save you from the Egyptians and from the Amorites, from the Ammonites and from the Philistines?