The Bowser UnWorld Territories (or BUT) are a collection of African tribes that fight in the name of Bowser and part of the Bowser Empire. They were first captured in the -1 through 2 centuries when the slave trade was first starting. Bowser wanted to "protect" the Africans from slavery, so they allowed him to become their unquestioned and unchallenged king (this was before Dry Bones was made and he became the Koopa King). He has since made them all his slaves, and they are forced to serve him as he takes over the UnWorld.
When these territories were captured, Bowser was finally able to escape persecution in the Mushroom Kingdom and live peacefully in Africa. That way, Mario would have to travel farther if he wanted to rescue Peach. More importantly, Toad would be too lazy to travel all that way just to say "Your princess is in another castle." The Bowser UnWorld Territories still serve this purpose, though Bowser's base of operations has since moved to North Hyrule.