- (Australia, obsolete) Imaginary king of the Australian aboriginal people.
- Tasmanian aboriginal man William Lanne (c.1835-1869) believed to have been the last Tasmanian aboriginal man.
- Any one of various other aboriginal men at times referred to as King Billy.
- (Ireland, Scotland, informal) King William III of England.
- “King Billy”, entry in 1970, Bill Wannan, Australian Folklore, Lansdowne Press, 1979, ISBN 0-7018-1309-1, page 333.