The crossword solution for the term RELATIVE OF REX OR ROVER is FIDO, which is a four-letter word. FIDO is a common name for a dog, and is often used to refer to any generic dog. It is also a popular name for a pet, as it is short and easy to remember. FIDO is derived from the Latin word 'fidelis, ' which means faithful or loyal. Therefore, FIDO can be used to refer to a relative of Rex or Rover, such as a sibling, cousin, or other close relative.