The Bureau of Land Managment, California has this on their website
BLM California manages 15.2 million acres of public lands, nearly 15% of the state's land area. The Bureau also administers 47 million acres of subsurface mineral estate underlying federal surface land, 2.5 million acres underlying privately owned land, 592,000 acres of Native American tribal land where BLM has trust responsibility for mineral operations. BLM California also manages 1.6 million acres in northwestern Nevada for cattle grazing, wild horses, recreation and wildlife habitat.
there's a new documentary that tells another story called Last of the Spanish Mustangs
Good film, and you can rent it from netflix