You have intergalactic starships, but you don't have goddamned pants? How does a civilization's evolution just skip over that part?
Works that combine humans with non-human characters will often have the non-humans walking around stark naked (or as close as the Moral Guardians will allow) as they go about their everyday business. Genitals are never shown; at best, these beings might wear a Seashell Bra or Fur Bikini to avoid Squicking their human companions.
While they will seldom mention any discomfort, savvy viewers might wonder how they regulate their body temperature or protect against environmental dangers, especially if they lack fur, feathers, scales, or something comparable. If the topic is raised, it'll usually be justified as either a religious or cultural norm, or Bizarre Alien Biology.
One might also wonder about the lack of pockets, though this might also be an issue for normally dressed humans in some settings. Sometimes the aliens will point out that they have Outgrown Such Silly Superstitions, which might raise more questions about the aforementioned pockets.
Functionally, this helps to emphasize the otherworldly nature of the characters, as it contrasts the awkwardness of undressed humans against the easy comfort of the naked aliens/fairies/demons/whatever. It's also an easy excuse to provide some Fan Service or comedy, though (sadly) almost always averted with Human Aliens.
- Just about every "Grey" alien ever. In some media, though, they wear skin-tight suits.
- Teen Titans: According to Starfire, Tamaranians don't place much value in clothes (they see nothing inherently unchaste about nudity and they're pretty much Flying Bricks, so Armor Is Useless). This is largely her justification for being both an Innocent Fanservice Girl and Ms. Fanservice.
- Babe in the second Atari Force series goes around wearing nothing. Justified as he's from a species that grow into mountains, and his "skin" has the consistency of granite.
- In all of their incarnations, the Teenage Mutant Ninja Turtles are seldom seen wearing anything besides their bandanas and weapons, unless it's part of a disguise.
- Martinex, of Marvel Comics's Guardians of the Galaxy, is a crystaline alien from the planet Pluto. His body is composed entirely of crystal, and never wears clothes.
Films -- Live-Action
- In Bill and Teds Bogus Journey, the Station aliens are always naked, despite having furless skin and minimal body hair.
- ET the Extraterrestrial is another naked hairless alien.
- The alien family in Mac and Me, until they become American citizens at the end.
- Star Wars
- Chewbacca is probably the most well-known example, though he does solve the pocket problem by wearing a Badass Bandolier.
- Hutts are always shown undressed, despite a lack of natural protective covering. Hutt hides are thick enough to shrug off blaster fire, though.
- Averted with Paul from the 2011 comedy, who wears cargo pants hiding a Gag Penis.
Paul: What? This is small where I come from!
- Almost every alien in Men in Black, save for the ones with a Mobile Suit Human.
- The aliens in Signs. Becomes egregiously stupid when you realize that water is toxic to them. Although the aliens' natural camouflage would be defeated by clothes (which still doesn't make it any less dumb).
- The creature in Super 8.
- Possibly its garments were confiscated for analysis by its human captors.
- The aliens from Cowboys and Aliens only ever wear their gun-bracelets. It helps that most of their skin is Immune to Bullets, though.
- The movie version of The Lion, the Witch and the Wardrobe averts the matter of topless centauresses by putting them in (torso-only) armor.
- Pretty much all the aliens in Animorphs, particularly the Andalites, who first thought that human clothing was part of human's bodies. Ax has trouble understanding why he has to wear clothing while morphed as a human, and frequently refers to clothes as "artificial skin" and shoes as "artificial hooves".
- Lampshaded in one of the Sector General novels, when Dr. Conway muses that he should be able to easily find the Earth Humans among the crowd at the space station's "beach" by the fact that only they would be wearing clothing.
- The Martians of Edgar Rice Burroughs' novels are confirmed nudists, despite the harsh conditions on Barsoom. Dejah Thoris, in her first appearance, is described in these immortal words: "She was as destitute of clothes as the green Martians who accompanied her; indeed, save for her highly wrought ornaments she was entirely naked, nor could any apparel have enhanced the beauty of her perfect and symmetrical figure."
- In Turtledove's Worldwar series of books, the Race never wears clothes. Their idea of proper attire is to cover themselves with body paint. Somewhat justified in that the planets they had visited before were very, very hot, like their home star. Example: their polar regions on Home (the Race's home world) are famous for going down to sixty degrees Fahrenheit. They are utterly unprepared for Earth's far chillier climate, and suffer greatly in their campaigns in the USSR and America during the winter months.
- Centaurs in Piers Anthony's Xanth series take pride in being nude, they see it as having Outgrown Such Silly Superstitions and generally confirming their superiority to the rest of the world. One side character in one of the books has to take on this attitude after having gone through a transformation in order to be with her Centaur boyfriend, with limited success.
- ALF is smart enough to crack wise with the Tanners, but can't be bothered to put on a pair of pants.
- Stargate SG-1. The Asgard are highly advanced grey space aliens, but never wear clothes. They quite visibly have no genitalia, and since they reproduce through cloning have no need of it. Lampshaded when both Sheppard and Mitchell first meet them, and their primary concern is if they are supposed to walk around naked.
- "Grey" aliens in The X-Files exhibit this. Except for the episode "Unnatural", when the Grey is wearing baseball clothing.
- Odo from Deep Space Nine is technically naked most of the time, but since he's a shapeshifter it looks like he's wearing clothing and it's never really commented on.
- Slitheen from Doctor Who and The Sarah Jane Adventures, when not in human suits. They even rejoice in their nakedness.
- Cole in Tracker, who had a hard time with clothing, and staying dressed in the beginning, probably related to Cirronians being energy beings.
Myths & Religion
- The character Cosmo from the old MS-DOS game Cosmos Cosmic Adventure fits this trope to a tee.
- See the centaurs bit above? In Accidental Centaurs, when encountering civilization for the first time, Alex learns at his expense that not covering one's upper half is considered indecent exposure for males too.
- The Uryuoms from El Goonish Shive, being shapeshifters, can simply "delete" their private parts when not in use. However, none are seen naked outside of Imagine Spots, when it's also specified they do have use for clothing.
- Roger from American Dad, when he isn't playing dress-up.
- Gwen's grandmother in Ben 10 Alien Force. Of course, her entire body is luminescent. This actually seems to be the true form of all Anodites.
- Frisky Dingo: Killface, the Villain Protagonist, doesn't wear any clothes and few people seem to mind. Played with at the last moment of the series, when Killface's family, dressed royally, comes to Earth on their spaceship. His mother gets mad at his lack of clothes.
- Rivesh Mantilax and Seruba Velak, from the Doctor Who animated serial Dreamland.
- The Transformers, as a race of living Transforming Mecha, rarely wear clothes... though occasionally they choose to put some on, for whatever reason.