In the Pokedex, it's stated that Yungoos and its evolved form Gumshoos aren't native to the Alola region but imported from another country. It's implied that they were brought to Alola to get the Rattata and Raticate population under control but they rarely see each other because Yungoos and Gumshoos are diurnal while Rattata and Raticate are nocturnal.

This is a reference to when the state of Hawaii had a problem with Polynesian rats eating up sugar cane crops in the 1800s. They imported Small Asian Mongooses to kill the rats. However, while the rats were able to adapt to being strictly nocturnal, the mongooses stayed strictly diurnal. Because of this, both species have become a major problem in Hawaii.