The Cloud Foundation says many of the horses appearing in an upcoming documentary on the Pryor Mountain horses will no longer be free.
Volunteer executive director Ginger Kathrens says 57 of the 188 horses who lived in the mountains on the Montana-Wyoming border, now wait in dusty pens in 90-degree heat at the foot of the range.
The horses are now branded, with ropes and numbers around their necks.
Nineteen-year-old Conquistador is no longer a band stallion, the foundation says. He is number 5336. Grumpy Grulla, 21, is no longer a lead mare. She is number 5321.
The horses will be put up for adoption on September 26 as part of a national adoption day aimed at rehoming 1000 wild horses under the care of the Bureau of Land Management.
"They are losing what they value most - their freedom and their families," says Kathrens.
"Despite a national outcry and letters from Congress demanding that the bureau halt these roundups until an acceptable long-term plan is made, we have yet to see them make a single concession to an outraged public.
"Somewhere along the line the bureau forgot that these are the public's horses on the public's land."
The foundation said hope for change lies in the Restore Our American Mustang (ROAM) Act, now before the Senate Energy and Natural Resources Committee.
Advocates are planning a gathering, 'Mustangs on the Hill", in Washington D.C. on September 29.
Supporters keen on preserving wild herds will fan out and meet with key Senate staffers and Senators.
"The ROAM Act comes too late for thousands of horses, but we are hopeful that Congress can ride to the rescue for our wild horses," says Arizona advocate Julianne French.
"Isn't it time that the public finds out the truth, that this gross misconduct is not a result of managing for healthy horses on healthy rangelands, but is an all-out eradication of America's wild horses?" asked advocate Monika Courtney.
"The bureau is betraying not only our horses, but our nation."
As one advocate stated: "The West will one day be about as wild as Wal-Mart."
The small Spanish mustangs in the Pryor herd, descendents of the Lewis and Clark expedition horses and the original Crow war ponies, may not be aware that their highly contested roundup and subsequent removal has created a wave a protest.
"A whole new group of advocates concerned about our wild horses have come out against this roundup," says Willis Lamm, a horse trainer noted for his work with bureau mustangs.
"Moving forward with this roundup was a huge mistake on the part of the bureau."