Metal Gear Solid 3: Snake vs. Monkey / Ape Escape 3: Mesal Gear Solid
Original genre: Stealth Action
Spin-off: Action Stealth
What does a ridiculously complex spy series and a lighthearted monkey-catching franchise have in common? Not much. That didn't stop Konami from outfitting Metal Gear Solid 3: Snake Eater with Snake vs. Monkey, a minigame which tasked the master infiltrator with heading into the jungle to capture wayward monkeys from the Ape Escape series. Sony responded in kind with its own Mesal Gear Solid add-on in Ape Escape 3, a minigame which followed Pipo Snake as he battled monkey-esque Metal Gear on a mission to save Solid Snake himself.
Granted, neither Metal Gear Solid or Ape Escape were ever known for making sense, but this crossover was weird by even their standards. Snake vs. Monkey was later dropped for the Metal Gear Solid: HD Collection and Metal Gear Solid: Snake Eater 3D, meaning you're going to have to hit the retro shops to enjoy this odd (but strangely charming) spin-off.
Expect the unexpected
Those are our choices for the strangest spin-offs in all of gaming--but perhaps you've got something even weirder in mind. Tell us about the strange side game we missed in comments, and remember, insane amounts of change can sometimes be a good thing.
And for more fun stories on GR+ head on over to Real-life video game weapons you can hold in your hands or try 10 best video game parody accounts to follow on Twitter.