Journey to the West is one of the most well-known pieces of Chinese literature. But I've always found it puzzling how Sun Wukong (aka Monkey or the Monkey King) became associated with the historical monk Xuanzang's journey to India. How did the story of Sun Wukong originate? How did it get wrapped up with Xuanzang's story?
There are two popular theories as to how "Monkey" became an integral part of the story. One is that as part of Indian Buddhist teachings, they incorporated some Hindu cultural stories, including the epic of Ramayana, where Rama, an avatar of Vishnu, has dealings with a Monkey kingdom, including killing the existing king in favor of an alliance with a claimant to the throne. As JTTW is a work with strong Buddhist and Indian themes, it seems likely that as an "Indian" story it would be appropriate to include.
The other is that Xuanzang met some barbarians along the way to India who called themselves the "Hutseng." A bit of clever bastardization and it sounds like "Houzi," the Chinese word for monkey.