Southwest of Encampment in the Medicine Bow National Forest, in the southwest corner of the Sierra Madres section of the MBNF, there is an epic ATV route. The trailhead access is on SR70 about 30 miles southwest of Encampment at Stock Drive (may not be signed).
The Snowy Range Mountains section of the Medicine Bow National Forest has miles and miles of forest roads open to ATV riding. Make sure you have the current Wyoming ORV permit.
Overall, Sierra Madres - Southwest is the 6th most popular ATV riding trail of all 27 ATV trails in Wyoming.