Wikipedia defines "posse comitatus" this way: The posse comitatus, in common law, is the group of people mobilized by the conservator of peace – usually the sheriff – to suppress lawlessness or defend the county. The posse comitatus originated in ninth century England simultaneous with the creation of the office of sheriff. Though generally obsolete throughout the world, it remains theoretically, and sometimes practically, part of the United States legal system.
Posses are by no means obsolete in the United States, and a few of them still ride horses when in service. They provide added security for events, help in searches for missing persons, and can be called on to aid the sheriff with traffic control, among other duties. Today, some of them drive departmental cars, SUVs, or pickups with far more horsepower and range of distance than a single horse can manage.
Nodaway County has had posse men off and on throughout its history, depending on the need at that time. Sheriff Tucker reestablished a posse during his term. They participated in community events, like rodeos, and helped keep things in check. Cass County has a sheriff's posse today, as does Greene County, and Bates County, just to name a few. Here's some information on the Jasper County Missouri Sheriff's Posse.
Today, 'posse' means something different in America's urbanized lingo, and has little to do with law and order, but is used to describe followers of the rich and famous, and sometimes infamous.
There are those who still uphold the integrity and tradition of the American posse as it was first established, and in some instances today they are referred to as 'reserve officers.' It's good to see this tradition carrying on, whether on four legs or four wheels, enabling the community at large to play a part in securing the county they represent.