 Yavapai County Mounted Sheriff's Posse. The Mounted Sheriff's Posse is the oldest Mounted Posse in Arizona. They were formed in 1941 and worked with the Sheriff's Office as aides due to the shortage of manpower and later became a search and rescue unit. Over the past years, the Yavapai County Mounted Sheriff's Posse has raised and distributed over $633,000 in scholarship funds to the youth of Yavapai County. And there they are.