You can get this after meeting Juan Cortez for the first time at the bar and bringing him back to Clara's Guerrilla Camp.
After a short shootout once you leave the bar with Juan, you'll get a chance to pet Guapo. You must pet him twice for the trophy to pop: Hold ■ once to pet him and you'll feed him some meat, then hold ■ a second time to actually pet Guapo and the trophy should pop while you're petting him.