Ammonites were descendants of Ammon and were closely related to the more civilized Moabites; Biblically their names were interchangeable.
The Ammonites were a nomadic race, their founder having dispossessed the Zamzummim (Deuteronomy 2:19-21) dwelt east and north of Moab, from the Arnon to the Jabbok. “Sihon the king of the Amorites” just before the Exodus took the land between the two streams from “the former king of Moab” (Numbers 21:26), “from the wilderness to the Jordan (Judges 11:22), and thus crowded Ammon eastward into the desert.
Although the Israelites were forbidden to molest the Ammonites, Ammon
was frequently in league with other nations against the Israelites. They
were usually hostile, before and after captivity. Their hostility continued until they were swallowed up by Rome, then about 150 AD they became numerous again until about 186-254 AD when they were merged with the Arabs.
The Ammonites were governed by a king (1 Samuel 12:12). The national
deity was Molech (1 Kings 11:7), often called Milcom (1 Kings 11:5, 33). Their capital was called Rabbath or Rabbath Ammon, which was revised several times to become Amman. They helped Syria and was among the twelve kings of Hatti.
Unger, Merrill F., Unger’s Bible Dictionary, Chicago, Moody Press, 1966, p. 45