Who were the Ammonites?

Ezra 9:1 (ESV)

1 After these things had been done, the officials approached me and said, “The people of Israel and the priests and the Levites have not separated themselves from the peoples of the lands with their abominations, from the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians, and the Amorites.

The Ammonites were descendants of Lot by an incestuous union with his younger daughter (Genesis 19:38). They lived in the area around Rabbath Ammon.