The aliens look and act like humans. But when nobody’s looking - or when the dramatic moment arrives to reveal themselves - they morph into their true form, be it reptilian or insectoid or some other bizarre physiology. I feel like I’ve seen this enough for it to have become cliche, yet I’m struggling to come up with any specific examples.
The crew from Third Rock From The Sun assumed human form to infiltrate Earth. If I remember correcty, this transformation was a difficult process, and they were pretty much stuck that way for the duration of their mission. The pods in Invasion of the Body Snatchers required a human blueprint of sorts to create a replica that looked, but didn’t really act, human. Neither of these examples is quite what I’m talking about.
What I’m thinking of is the alien that can shape-shift back and forth from human to alien form at will (and what a remarkable ability for any species to have evolved!). What examples have you got?
Star Trek’s Changelings - While Odo never really mastered humanoid faces, other Changelings became perfect replicas of humans, Klingons, Cardassians, Romulans, and other races.
Similarly, the Durlans of DC comics. Their natural form is never shown clearly, but is highly tentacular, and they have a second default form they use when around other races, but not disguised as them, where they’re orange-skinned bald humanoids with antennae. They have at various times used that ability to blend in, or infiltrate other races. (Olivia Marsdin from the Supergirl TV show, and RJ Brande from the Legion of Superheroes comics are Durlans who spent significant amounts of time in human form - Brande was actually stuck in that form for most of his history. Marsdin’s default form doesn’t resemble either of the comics ‘Durlan’ forms, probably because the tentacular form would be hard to pull off and the orange humanoid would look a bit silly in live action.)
DC also has several Martian races with the ability to shapeshift, including human - Green Martian J’onn J’onzz and White Martian M’gann M’orzz use it to have secret identities (John Jones and Megan Morse) - M’gann also has a false Green Martian form to avoid being judged for being a White Martian - other Whites frequently use it to infiltrate.
The Piurivar (aka Metamorphs/Shapeshifters) of Robert Silverberg’s Majipoor books come to mind. They get annoyed by sentient beings from other worlds settling on their planet and displacing them, and use their abilities to impersonate various ruling figures.
Nope. Fails the OP criterion “What I’m thinking of is the alien that can shape-shift back and forth from human to alien form at will.” Dire Wraiths have to suck the essence from a human to be able to take their shape.
Hank Henshaw is badass enough in the first season of Supergirl, but every once in a while, his eyes glow red (out of sight of everyone but the camera, and often at the end of an episode, so you’re watching and you yell, “Oh, come onnnnn, we want to see what happens next!”)
Turns out, he’s an alien that can shape-shift back and forth from human to alien form at will.Here’s the big reveal, with a bonus whammy at the end.
The writer and actor discuss it here.
Various aliens in the Men in Black films had the ability to either take human form or had the ability to disguise themselves as humans using various methods (with varying degrees of effectiveness).