Little Green Man in a Can

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 butts 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 Armour and similar things.

May overlap at times with Mobile Suit Human if the "can" is human shaped. Contrast with Exposed Extraterrestrials.

Film

 * The Emperor 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.
 * 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 Will Smith's character knocked one out with one punch.
 * The Skakoans from Star Wars 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.

Literature

 * The One from Last Legionary is a mutated human with stunted limbs, wearing a massive golden exoskeleton that turns him into The Juggernaut.
 * The Creapii from Terry Pratchett's 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)

Live Action TV

 * The Daleks (pictured above) from Doctor Who, though their suits aren't so much suits as little tanks in which utterly feeble bodies reside.
 * The Ice Warriors, too: their big, hulking bodies are stated to be biomechanical spacesuits in their debut story.
 * The Vorlons from Babylon Five, who not only need a completely alien atmosphere but also have their own reasons for wanting to hide their true appearance. Comes complete with Neglectful Precursor behaviour.
 * They don't actually need that atmosphere, they just use it to discourage visitors.
 * The Gaim from the same series wear encounter suits that resemble humanoids in gas masks. Underneath, the 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.
 * 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 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 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.
 * Though according to Weyoun, their planet is actually quite mild. It's also said that if you saw a Breen in the flesh, it'd turn you to stone. However, this is also clearly a lie, as Kira manages to steal one of their uniforms without any ill effects.

Tabletop Games

 * Most Tau warriors from Warhammer 40000, with suits ranging from ordinary armour to Power Armour to Mini Mecha, although their spiritual leaders go bareheaded. They can all breathe our atmosphere unprotected, however.
 * Also, Ork Killa Kanz, which are literally little green men in cans.

Video Games

 * The Quarians from Mass Effect wear them to protect their absent immune systems from having to deal with normal environment. From a more Doylist viewpoint, it helps make the Space Gypsies seem more hidden and closed off from the other races.
 * Quarian spacesuits don't really leave much to the imagination, but the volus from the same series could look like anything under their bulky (or not?) suits. They even seem to have completely robotic "hands".
 * You can just barely see hints of Tali's face through her helmet.. but they're so vague that they do little more than establish that it has a human-like configuration.
 * The Unggoy (aka Grunts) from Halo. They are most often seen with masks on, large tanks on their backs. (They breathe methane on their home planet, and carry it with them much like divers and astronauts.) Some higher-ranking Grunts may also have other armor on in addition to the methane tank, or the tank and mask may look different (as with the Heretic Grunts.)
 * The Meklar from Master of Orion were originally this. As the series progressed, they gradually became Cyborgs, then full-on Mechanical Lifeforms.

Webcomics

 * 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.