Ditto with the Coromandel Grey Langurs in Theppakadu, Mudumalai. I have also commonly observed chital and sambar feeding on lantana leaves, and the JLR naturalists at BR Hills tell me even elephants feed on it. I doubt if lantana actually is poisonous, though. Langurs have amazing digestive systems (AFA digesting leaves is concerned) able to digest some of the most unpalatable leaves.

Have you tried tasting L. camara leaves? The mature ones are quite tasty, a bit like tulsi and a bit like mint. In Arunachal Pradesh (Jairampur and Miao) many people use the crushed leaves and leaf juices to treat cuts and bleeding wounds. It is believed to disinfect and also coagulate the blood.