Seventh Doctor: No, little green blobs in bonded polycarbide armour.
It's something that tends to happen with particularly advanced mentor-like alien species, Sufficiently Advanced Ancient Astronauts and the like. You don't want to be something as uninspiring as a Rubber-Forehead Alien, nor something as unphysical as an Energy Being while also maintaining some sense of secrecy and wonderment, so you take the "astronaut" idea and run with it. The aliens end up wearing large all-enclosing hazmat-like encounter suits that shield their frail physical selves from the elements.
The great thing is that because you don't know what their encounter-suited butts look like, they could be anything! The variety of abnormal suit shapes that can be worn by a performer is also greater than the variety possible with rubber faces. People in Rubber Suits still need the full detail and design of a whole new species, while this trope leaves things to the imagination. A permanent life in an encounter suit can also be justified a lot more than Humanoid Aliens: they have Bizarre Alien Biology that can't stand our atmosphere, they're physically very frail by comparison, so it works like Powered Armor and similar things.
- The aliens in Earth vs. the Flying Saucers fit this trope to a T. They wear bulky Powered Armor while being little green men (well, little GREY men, but that's black and white film for you).
- The Mondoshawan from The Fifth Element have really big waddling rears with tiny dog-like heads that give them a very odd Humpty Dumpty appearance.
- The aliens from Independence Day. The suit itself is explained as being some kind of Organic Technology, and indeed, the helmet portion looks like it's a solid external skull. This makes it retrospectively more badass that Hiller knocks one out with one punch.
- The prince of the Arquillian Galaxy from Men in Black, of course. He's the classic Little Green Man in a can... disguised as an elderly Eastern European jeweler.
- Star Wars: Skakoans such as Wat Tambor are required to wear a full body environmental suit as they live on a planet with a high pressure atmosphere. Going to a "normal" planet unaided would cause explosive decompression, like bringing a deep sea fish up to the surface.
- Krang in Teenage Mutant Ninja Turtles: Out of the Shadows is basically a brain with a face and tentacles who lives in a bulky set of Powered Armor. This also applies to his animated incarnation as well.
- The Creapii from The Dark Side of the Sun are always encountered in tentacled, ovoid environmental suits. Not because they're too frail to survive the kinds of atmospheres the other sentient species use, but because their natural habitat is the photospheres and upper corona of stars (with a class system based on how hot their home star is). Their true form is said to be spiral-shaped, with the result that their mythology anthropomorphised the galaxy as a giant Creap.
- The One from Last Legionary is a mutated human with stunted limbs, wearing a massive golden exoskeleton that turns him into The Juggernaut.
- The Ur-Example is probably the Martians from The War of the Worlds (1898). They're actually fairly big, but ill-suited to Earth's stronger gravity and have a hard time getting around on their own. Therefore, most of the invasion is enacted through their "Fighting-Machines", a force of enormous mechanical tripods, and a handful of other more utilitarian vehicles. It's suggested they switch to different machines for different purposes in the same way a human would switch tools for different jobs.
- Babylon 5:
- The Vorlons, who tell the Earth Alliance they need a completely alien atmosphere but also have their own reasons for wanting to hide their true appearance. Kosh tells Sheridan that if he left his encounter suit he would be recognized by everyone. Later in the series it's revealed that they don't actually need that atmosphere and can function perfectly well in a standard atmosphere. When Kosh leaves his encounter suit to save Sheridan's life he is indeed recognized by everyone. The Vorlon behavior comes complete with Neglectful Precursor behaviour.
- "The Deconstruction of Falling Stars" shows that in the distant future humans become like this too, though they don't need to; they probably took a lesson from the Vorlons about the advantages of standing around being cryptic when dealing with younger races.
- The Gaim wear encounter suits that resemble humanoids in gas masks. Underneath, they are insectoid and apparently far from humanoid. The suit serves both as environmental protection and a translator/speaker unit, as the Gaim are incapable of vocalizing most species' languages. As a hivemind species, Gaim units are genetically engineered by their queen for specific tasks. Thus, the Gaim ambassadors and traders seen on Babylon 5, needing to interact with the mostly humanoid races, are more humanoid than typical for their species, even being granted a degree of autonomy and individuality.
- Doctor Who:
- The Daleks (pictured above), though their suits aren't so much suits as little tanks in which utterly feeble bodies reside. In the Seventh Doctor's words, they're not so much little green men as "little green blobs in bonded polycarbide armour". This varies with the story; in "The Daleks", "The Space Museum", and "The Witch's Familiar", it's such a simple mechanical device that a humanoid pilot can climb in and drive it around, while "Into the Dalek" (a season before the latter) depicts the suit as a fully-integrated cybernetic system that artificially manipulates the organic part to prevent it turning against its programming.
- The Ice Warriors, too: their big, hulking bodies are stated to be biomechanical spacesuits in their debut story. However, in "Cold War", it turns out that Ice Warriors are anything but frail outside of their suits; they stay in the suits primarily for cultural reasons related to honour, and also because their military prefers Mighty Glacier to Fragile Speedster.
- The Krotons are an especially weird variant, even by Doctor Who standards — they are a form of semi-living crystal dependent on their Tin-Can Robot suits to have useful physical form. They need to breathe from a "polarised centrifuge" to remain in this living state, but the closest thing to an actual death they have is just "exhaustion", which is when their crystals revert to a simple (gaseous) state, and which is apparently temporary. In the Eighth Doctor Adventures, the Krotons' implied ability to inhabit any machine is made explicit when it is revealed they are able to possess Dalek ships.
- The Delegate from Arcturus in "The Curse of Peladon" is dependent on a huge black life support system out of which only his head is visible, enclosed in a solid clear dome that constantly spurts fluid. Some of his technology includes a sensor array, without which he becomes comatose, and a 'helium regenerator' which sabotaging would make the easiest way to kill him. Due to how much it restricts his movement and capacity for doing anything, he crosses over into Evil Cripple.
- In Stargate Atlantis, the team encounters a group of aliens who kidnap Daniel (who's in the galaxy visiting) and Rodney. It's eventually revealed that they're a group of rogue Asgard who have taken the degenerative cloning problem into their own hands and are using man-sized suits to exist on a poisonous planet with no stargate as a means of staying hidden.
- The Breen of Star Trek, who first appeared on-screen in Star Trek: Deep Space Nine, come from a much more frigid environment than most humanoids and need environmental suits to interact with other races face-to-face. It's later stated in Star Trek Novel 'Verse that there's more than one kind of Breen, they just all wear environment suits that make them look identical. Some of them need the refrigeration, some of them don't.
- Destiny: The Vex are an interesting take on this trope. The bipedal members of the race fought throughout the game — Goblins, Hobgoblins, and Minotaurs — seem entirely robotic in appearance, possessing a glowing white core on their abdomens that explode into milky fluid when destroyed. Said fluid, however, is not Machine Blood, but what the Vex actually are: colonies of microscopic, hyperintelligent protozoa in a nutrient soup capable of piloting mechanical shells.
- The Unggoy (aka Grunts) from Halo are a partial example. They tend to wear no clothes and only partial armor, but they're always seen with breathing apparatuses on their mouths/nostrils and large tanks on their backs, as they can only breath methane and have to carry it with them much like divers and astronauts.
- Mass Effect:
- The quarians wear suits to protect their absent immune systems from having to deal with normal environment. From a more Doylist viewpoint, it helps make the Romani Fantasy Counterpart Culture seem more hidden and closed off from the other races. Only a borderline example, however, because the space suits are fairly form-fitting and the quarians themselves seem to be more or less humanoid. It's possible to see a photo of Tali, a major quarian character, with her mask off in the third game if you play her romance, and she looks like a normal human with white irises, although her limbs and hands are quite different.
- The volus also wear suits, because they suffer from Explosive Decompression and breathing problems outside their native high-pressure and ammonia-heavy atmosphere. They are a more traditional example, as they could look like pretty much anything in there.
- The Meklar from Master of Orion are originally this. As the series progresses, they gradually become Cyborgs, then full-on Mechanical Lifeforms.
- The evil Skedar in Perfect Dark. We never see their tiny true forms, but only the bipedal, roughly human-sized exosuits they use to get around and fight.
- The K'Tang from Star Control 3 are basically grubs with a major inferiority complex, so they talk big and wear giant suits of power armor to compensate.
- The Tholians in Star Trek Online can only survive in environments like the surface of Venus, so humans have to wear encounter suits when fighting them on extremely hot worlds. The Tholians likewise, will use their own encounter suits◊ when fighting humans in temperate environments.
- The Andromedons from XCOM 2 wear encounter suits to maintain the weapons-grade acidic environment they need to survive in. If you kill an Andromedon, it isn’t the end of their threat, as the ruptured suit reactivates on its own and tries to smash your troops while leaking acid everywhere.
- Sam Starfall from Freefall. He wears an environment suit, but we've never seen his actual form. It's implied that it's so horrible that it destroys the minds of anyone who sees it. Also, his suit is deliberately designed to be cute, as a means of distracting people so he can steal from them.
- In World's Greatest Adventures, the Martians appear to be this — Cassius clearly is, at least.
- This trope turns up often in Men in Black: The Series, crossing over with Mobile-Suit Human. In one episode, there's even a Russian Nesting Doll of them — a little green man piloting an android, who himself is a robot piloted by a little blue man!
- Star Wars: The Clone Wars: One of the title characters in "Bounty Hunters", Seripas, turned out to be a small alien in Powered Armor.