Futurama's executive producer David X. Cohen stated in the audio commentary that the title refers to the phrase for sexual intercourse: "the beast with two backs," popularised by William Shakespeare. Incidentally, this phrase was also referred to in the episode "Spanish Fry" where it was the name of a sex store: "The Beast With Two Bucks." Also, "The Beast" is a reference to Yivo, and the "Billion Backs" is refering to all the humans he captures with his genticles.