the Ammonites were enemies of the Israelites in Old Testament times.
True - but then, so was just about everybody else in the region, including Kenites, Kenizzites, Kadmonites, Hittites, Perizites, Rephaim, Amorites, Canaanites, Girgashites, Hivites, and Jebusites (Genesis 15, 19-21). To say nothing of Moabites and Edomites. I think the trouble was (as so often is the case) that there were rather more people than there was land to accommodate them all (plus ça change, plus c’est la même chose) The Ammonites were, in fact, cousins of the Children of Israel, being descended from Abraham’s relative, Lot. Their territory was East of the Jordan. The capital of present-day Jordan, Amman, takes its name from them.
The fossil Ammonites get their name from their shape, which vaguely resembles a curved ram’s horn (the Egyptian god Amun, or Ammon, had a ram’s head). Ram’s horns are often to be seen (as divine attributes) on coin-portraits of Alexander III “the Great”, King of Macedon