Ensign, June 1977, pg. 73; “The story of Ammon is one of the most dramatic and heroic in scriptural literature. From a rebellious prince, one of “the very vilest of sinners” (Mosiah 28:4), fighting against the church of God in his father Mosiah’s kingdom, Ammon underwent a transcendent conversion and dedicated the next fourteen years of his life to a perilous and difficult mission among the Lamanites. With his brothers, Ammon was instrumental in converting thousands of Lamanites to Christ, thereby changing the course of Lamanite history. At the end of his mission, Ammon became the high priest over these same people (see Alma 30:20), who were called after him—the people of Ammon (see Alma 27:26).” – Robert A. Rees
Genesis 19:36-38; “Thus were both the daughters of Lot with child by their father. 37 And the firstborn bare a son, and called his name Moab: the same is the father of the Moabites unto this day. 38 And the younger, she also bare a son, and called his name Benammi: the same is the father of the children of Ammon unto this day.”
So – the Ammonites are the descendants of an incestuous relationship between Lot and one of his daughters. To find out more information about the behavior of these people see Amos 1:13-15 and an article on our site about demonic names in the Book of Mormon. http://www.lifeafter.org/demonic_names.asp
More than a few of Solomon’s concubines were Ammonites and they were, in part, responsible for turning Solomon’s heart away from God – they were worshipping Milcom.