From: http://nativeamericans.com/Arapaho.htm
Native North Americans of the Plains whose language belongs to
the Algonquian branch of the Algonquian-Wakashan linguistic
stock. Their own name was Inuna-ina (our people), but they
were referred to as dog eaters (for the obvious reason) by
other Native Americans. Tradition places their early home in
N Minnesota in the Red River valley, but nothing is known of
the date or circumstances of their separation from other Algonquian
peoples. They are thought to be most closely related to the
Cheyenne and to the Blackfoot. However, it is known that the
Arapaho divided into two groups after they migrated to the
plains. One group, the Northern Arapaho, continued to live
on the North Platte River in Wyoming, while the Southern Arapaho
moved south to the Arkansas River in Colorado. Traditionally
the Southern Arapaho were allied with the Cheyenne against the
Pawnee. The Arapaho placed some emphasis on age grades, mainly
for ceremonial purposes. Their annual sun dance was a major tribal
event, and later the Arapaho adopted the Ghost Dance religion.
There are three major divisions—the Atsina or Gros Ventre,
who were allied with the Blackfoot and now live with the Assiniboin
on the Fort Belknap Reservation in Montana; the Southern Arapaho,
now living with the Cheyenne in Oklahoma; and the Northern Arapaho,
who retain all of the sacred tribal stone articles and are considered
by Native Americans to represent the parent group. Since 1876 they
have lived on the Wind River Reservation in Wyoming with their
former enemies, the Shoshone. See G. A. Dorsey and A. L. Kroeber,
Traditions of the Arapaho (1903, repr. 1974); V. C. Trenholm, Arapahoes,
Our People (1970).