The common name is Porcine Zona Pellucida, a protein derived from pig ovaries which interferes with conception.
According to the EPA Pesticide Fact Sheet, it’s a tool for management of nuisance feral and wild horses and burros. The technical registrant is the Humane Society of the United States.
The substance is injected intramuscularly by hand-held syringe, syringe mounted on a jabstick or by syringe dart fired from a CO2 cartridge-powered projection system.
The goal is to have fewer of these little guys running around on the range. You have to wonder how anyone who agrees with this can be called a wild horse advocate.