Retractable Appendages

Exactly What It Says On the Tin, The ability to retract one's body like a turtle or snail. Your Mileage May Very but it can be considered either Nightmare Fuel or Extremely Awesome when a Human has such an ability.

Comic Books

 * Mr.Fantastic can retract everything, turning into a sphere. That is how he survived exposure to space once.

Films

 * Lilo and Stitch has Stitch with retractable antennai, back spikes and 2nd pair of arms.
 * Almost all of Stitches "cousins" have this ability.
 * Inspector Gadget 2 has this happen to Gadget when he ducks a punch in the face at a bar.
 * In The Hitchhikers Guide to The Galaxy, Zaphod retracts one of his heads into his chest from time to time.
 * Kaiju movies starring Gamera, the flying turtle. Gamera can retract all four of his legs into his shell, then cause fire to spray out of the holes, allowing him to spin around and fly. He can retract his head into his shell the same way, and often does so while flying.
 * Toothless from How to Train Your Dragon has retractable teeth hence his name.
 * An early Stephen Chow movie, The Royal Tramp, involved a battle with the Made of Iron villain who could even retract his manparts. But Stephen's character, having grown up in a brothel, learned how to counteract this by shoving something up the backdoor.
 * The electronic devices in The Brave Little Toaster all have retractable faces, with the exception of Radio, who for some reason actually doesn't have a face.

Literature

 * The Future Is Wild has the Flish with a retractable second mouth.
 * Slickribbons have the Flishes retractability brought Up to Eleven.
 * In the James Bond book You Only Live Twice Bond visits a Japanese martial arts training facility, where the fighters have been trained to retract their testes, making them invulnerable to a groin attack. You really do this (if you're a male), but to a lesser extent than they show in the book.

Tabletop Games

 * Dungeons and Dragons. Giant Snapping Turtles are noted as being able to retract their legs and head into their shell, thus improving their armor class.

Video Games

 * Sephiroth from Kingdom Hearts has a retractable black wing. Implied in the Final Fantasy VII series that his black wing is the same as in Kingdom Hearts.
 * Genesis and Angeal from Crisis Core also have retractable wings.
 * Pokémon Generation V has the Golem like Golurk who is able to fly this way by retracing its arms and legs.
 * Any Pokemon with the ability Withdraw.
 * Also Darumakas in battle and in the anime can retract their limbs into their body, giving them the appearance of an egg.
 * One of Hoothoot's legs is retractable.
 * Darkrai also has retractable legs.
 * In Final Fantasy IV, the turtle monster Cagnazzo is a creepy example of this. He can collapse to invulnerable shell form, even though his beefy humanoid arms and legs are clearly too big to fit in his shell.
 * Kefka from the Dissidia: Final Fantasy series has six retractable wings that only detract when he uses the attack "Havoc Wing".
 * Algol from Soul Calibur IV has 3 on each side retractable kraken like clawed fingeresque tentacles that detract from his chest.

Web Originals

 * A fun image on Neopets reveals Cybunnies can retract their heads into their body.

Western Animation

 * Filburt from Rocko's Modern Life Justified that he is a turtle.
 * Gary from Spongebob SquarePants also justified knowing he's a snail.
 * Also happens to Craig Mammalton in the episode "Sun Bleached".
 * Also happens to Patrick in the episode "Not Normal".
 * Cat Dog has the titular character retract its body like a snail, there's only one episode where the play it rather... Gruesomely.
 * Anything involving Turtles and Snails.
 * Shendu from Jackie Chan Adventures normally has arms and legs, but for some reason he actually loses both after being turned into a statue thousands of years ago. He eventually gets his arms and legs back once he's been reverted back from his statue form.

Real Life

 * Turtles
 * Snails
 * Snail eyes.
 * The glans in the penis and clitoris is this.
 * The foreskin in the penis is this too.
 * The penis itself is this to most animals.
 * The testes are this partially, sometimes to greater extents in some animals.