Owyhee County was indisputably the first to be created by the Idaho Territorial Legislature in 1863. But Boise, Idaho, Nez Perce, and Shoshone were already counties when Idaho Territory was created. How so? They were created by the Washington Territorial Legislature when what we think of today as North Idaho was a part of that territory. Idaho lawmakers didn’t get around to officially recognizing them until 1864.
Owyhee County is big. Originally it was even bigger. The original county was all the land south of the Snake River and west of the Rockies. That lasted about a month before the Idaho Territorial Legislature took a big chunk of it for Oneida County. Today, puny little thing, Owyhee County is only the second largest county in the state at 7,697 square miles, behind Idaho County, which is 8,503 square miles. You could fit Rhode Island, Connecticut, and Washington DC into Owyhee County with 540 square miles to spare. But why would you want to? It would only irritate the jackrabbits.