Cassia County, Idaho was created on February 20, 1879, by an act of territorial legislation. At that time, the county included all of the territory south of the Snake River from Raft River on the east to Devil Creek on the west side of the city of Twin Falls which was near the boundary of Owhyee County. The first county seat of Cassia County was Albion. The county was named after Cassia Creek which derived its name from two words: cajeaur, peasant French for raft; or James John Cazier, a member of the Mormon Battalion and a colorful captain of an emigrant train, whose name was corrupted to Cassia. Most natives of Cassia County hold that the word comes from the “cassia” plant which does not grow along Cassia Creek.
More information can be found here:
Cassia County Historical Society