When it comes to cattle, Wyoming is a FENCE OUT State. That means if you live in rural Wyoming and you do not want cattle to roam freely on your land, then it is your job to fence them out! Isn't that a funny concept?!
Sheep, goats, swine and elk(?!), on the other hand, are required to be fenced in. Though, permissions are given that allow herds of sheep to be crossed from one grazing area to another by going through someone else's land if necessary. However, that is "only as long as the animals are not allowed to stop and de-pasture the enclosure." Good luck telling the sheep that!
To find out more about this curious custom but quickly and painlessly, Cole Ehmke wrote an excellent short article called "If You Fence It, They Will Stay Out" at: http://uwadmnweb.uwyo.edu/cesfrm/IfYouFenceIt.pdf.
Lastly, you can follow a lively, online discussion from 2007 at:
"Fence Out" is a concept I never heard before I got into real estate, but now that I know what it is, it explains A LOT about the gates and fencing we encounter in the public wilderness areas around here!