The Javan rhinoceros (Rhinoceros sondaicus) is one of the rarest large mammals on Earth. Once found throughout Southeast Asia, it is now critically endangered and confined to the Ujung Kulon National Park in Java, Indonesia. These rhinoceroses are solitary animals and primarily herbivores, feeding on leaves, shoots, and fruits. The Javan rhinoceros is distinguished by its single horn and thick, gray skin.
What is the main topic of the text?