Other than Oklahoma Meat Co.'s application, only five other companies have listed horses as animals to be slaughtered since November 2011, San Francisco attorney Bruce Wagman said.
Wagman represents Colorado-based Front Range Equine Rescue and requested the horse slaughter applications from the USDA in late January through an open records request.
Two of the companies are from Missouri. Others are located in Iowa, New Mexico and Tennessee.
A horse slaughter facility in New Mexico likely is the closet to actually processing the animals, but USDA officials said in a recent statement that its inspectors are still in the process of being trained.
Valley Meat Co. in Roswell, N.M., is suing the USDA in federal court, claiming the agency's inaction has cost its owner time and money. The suit, which is pending, is seeking to force the agency to resume inspections.
According to a June 2011 report from the Government Accountability Office, the number of neglected horses rose sharply after the federal ban in 2006.
Last year, more than 150,000 horses were shipped to Canada and Mexico for slaughter.
Contributing: Staff Writers Zeke Campfield and William Crum; The Associated Press