Final answer:
Halothane was a widely used inhalant anesthetic in veterinary medicine that is no longer available in the United States due to safety concerns and environmental impact.
Step-by-step explanation:
The inhalant anesthetic agent that was once very popular in veterinary medicine but is no longer available in the United States is halothane (2-bromo-2-chloro-1,1,1-trifluoroethane). Halothane has been replaced by less flammable, more stable, and overall safer alternatives such as desflurane, isoflurane, and sevoflurane. Despite its historical usage and effectiveness as an anesthetic, concerns regarding its safety, especially for operating room personnel, have led to the discontinuation of its use. Operating room workers exposed to halothane had a higher rate of miscarriages, and issues related to the compound's environmental impact ultimately contributed to its fall out of favor.