Officials at Theodore Roosevelt National Park plan to round up more than 100 horses this fall to thin the herd.
The surplus horses will be sold at auction on September 28th in Wishek.
The sale is being promoted via Facebook by people who want to make sure the horses are sold to caring buyers.
They say the last horse roundup in 2009 ended with eight of the 77 horses that were sold going to slaughterhouses.
In addition to the roundup, mares that are captured will be given a contraceptive treatment that may control the population without the need for future roundups.
A park official says the herd has grown beyond its ideal size and needs to be reduced.