It's actually a home to Ghosts. Toc-Man lives in his own castle, as well as the Ghost Gang, Clyde, Pinky, Inky, and Blinky.
Hence the name, it is home to any kind of ghosts.
Ghost (That look just like Clyde except its color is Green, Yellow, Purple, or Gray.)
Koopatrol
Boo
Dark Boo
Dry Bones
Bony Beetle
Wizrobe
Stalios
Mumbies