Otter National Forest was established by the U.S. Forest Service in Montana on March 2, 1907 with 123,779 acres (500.92 km2). On July 1, 1908 the name was changed to Custer National Forest.
The forest today comprises the Ashland Ranger District of Custer National Forest, in Powder River County and Rosebud County.
References
Otter National Forest Wikipedia(Text) CC BY-SA