That depends. Because Colorado is a fence-out state, a rancher in your area may have a grazing lease nearby where he pastures his cattle. The rancher is not obligated to keep the cattle fenced in. Individual landowners are obligated to keep the cattle fenced out of their property if they don’t want them on their land. In this case, Park County Animal Control will advise you to construct a fence to keep the cattle out (see the Grazing Ordinance for the definition of a legal fence).
There are some exceptions. Certain subdivisions are fenced all the way around their perimeter. Nine-Mile Heights, Pike-San Isabel Village, and Warm Springs are examples. If there are livestock in your subdivision and you believe it is fenced, Animal Control will attempt to locate the owner and can force that owner to move his livestock.