Mustang Info:
Nevada has the most wild mustangs, followed a distant second by Wyoming. Wyoming has around 7,000 horses scattered throughout 16 Horse Management Areas (HMA), a few parks, and the Pryor Mountain Wild Horse Refuge (spanning northern Wyoming and Montana).

Herds can swell by 15% to 20% per year and double within 4 to 5 years. Each HMA has a target range of horses and a set maximum. As the population reaches or exceeds that number, the BLM is required to hold a horse gather and remove the excess populations.

Many of the horses become available for adopton yearly. The younger colts and phillies are most likely to be adopted.

For more BLM information, click here.
(map borrowed from the BLM web site)