Agents assigned to the Nogales Border Station discovered three horses carrying over 1,000 pounds of marijuana worth an estimated $500,000 according to the news release. U.S. Customs and Border Protection reported that the horses were turned over to the Arizona Department of Agriculture.
"No matter how well you perform there's always somebody of intelligent opinion who thinks it's lousy." - Laurence Olivier
I've read some other news articles about this before, and I think it was also blogged about on fugly a while back.
From what I read the horses are used to get the bales of marajuana to locations where it can be loaded onto vehicles, which means they are packing heavy loads through the desert often with ill-fitting makeshift packs. Their welfare isn't exactly top priority to the drug runners and they sometimes get abandoned in the desert after they have served their purpose, left to die.