Mechanical Lifeforms are a race of robots or robot-like creatures that are also considered a honest-to-goodness species of living things. They're just like your everyday living organisms, except they happen to have metal for skin, wires for nerves, and so on. They're often silicon-based as well.
These may be robotic animals, plants, micro-organisms, or sapient creatures. If they are sapient, they would never wish to Become a Real Boy because, as far as they can see, they are as real as that boy.
The origin of such creatures is best left unexplained - they were never built by another race (well, that anyone knows of), and if they were, it would be treated as a very shocking revelation, due to the audience accepting their mechanical nature as-is. And should any creators arrive to cart them back, expect them to react just the same as humans would (i.e. much anger, denial, violence, and maybe a speech or two about free will).
Contrast Organic Technology, which are machines that happen to be organic in nature. Also see Mechanical Monster and Mechanical Evolution.
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The Mecha in Zoids don't just look like giant animals. They also live in the wild and somehow give birth as well. There's a picture book called the Zoids Bible which shows the zoids originally evolved from a planet seeded with zoid cores, going through eras of evolution uncannily similar to those of life on our own planet; the zoids as we see them on the show are post-domestication.
The Backstory is actually a lot more sinister. Natural wild zoid are mostly reasonably sized (sometimes they're large but nowhere as large as their current form), their giant mechanical body? It's manufactured specifically for war purposes, Wild Zoid are captured and transplanted on those robots as cores...
The characters from Machine Robo, which bizarrely has humanoid robots and mech-like robots in the same series.
The Rune-Gods from Magic Knight Rayearth. Though they're more like Energy Beings clad in suits of armor than robots. There's even an episode in season 2 where a pilot of a true Humongous Mecha tries to scan one of them and is baffled because he can't see any moving parts inside.
Tekkamen in their transformed states in Space Knight Tekkaman Blade seem to be actually made of Powered Armor. Occasionally, when it's dramatic, they retain scars into their human forms.
Digimon: The "Metal Empire" digimon includes cyborgs and guys wearing metal armor; nevertheless, their numbers primarily consist of full on robots, from the gear-like Hagurumon to the Giant Mecha given form, Machinedramon.
Of course, since they're all sentient computer data, all Digimon are this regardless of their form.
GaoGaiGar really messed around with this trope. First, there's Guy Shishioh, who's a traditional cyborg. The Zonders, Zonderians and 31 Primevals are also worth noting: the Zonders and Zonderians are organic beings that are transformed into some sort of techna-organic lifeform via exposure to Elementary Particle Z-O, which is released by Zonder Metal. They also have completely transformable bodies, and can assimilate metal, to the point where they can even move through it. The Primevals are the same. They just happen to have the ability to assimilate anything, not just humans. Then you've got Evoluders...
Special credit goes to the Zonderian, Penchinon: it is later revealed that, aside from being a Zonderian, he is actually the A-I system for the J-Ark.
The Neuroi in Strike Witches are a mixture of this and Starfish Aliens, as they take forms ranging from rockets and experimental aircraft to humanoid designs, flying manta rays and six-limbed turtles.
The villains in the new Galaxy Express manga and Eternal Fantasy are a race of these. (The old ones were mechanized humans.)
The titular Blue from Blue Drop. It's entirely mechanical and is (re)made of nanomachine, but it moves and acts like a mechanical whale.
The Mirrodin plane that the Planeswalker Karn created has plenty of these. Probably because Karn himself was one of these before he became a planeswalker.
Phyrexia was inhabited by mechanical lifeforms, even before Yawgmoth took control of it.
NYC Mech features an entire world populated entirely by robots, who look and act exactly like people.
At least one version of DC's Legion of Super-Heroes includes Robotica, a planet inhabited by a robot civilization, as well as the Linsnarians, a species of techno-organic people. In the cartoon version, the Coluan race (organic in the comics) is a civilization of humanoid robots not unlike the Linsnarians.
Marvel has the Phalanx (and a related race, the Technarchy), which has recently been taken over by Ultron, easily the most evil robot on Earth. Additionally, the Transformers sort of exist in Marvel continuity as part of their own timeline.
At least one of them exists in Empowered's verse. She has the shape of an attractive human woman and is anatomically correct. One of the Superhomeys sleeps with her, and her nanites turn him into a mecha.
Fantastic Four: The Thing once befriended a robot named Torgo from the planet Mekka; Mekka's organic population had died out in a disaster long ago, but their robot servants had survived and gone on to build a civilization of their own.
Warlock's people the Technarchs presumably started out getting built by somebody, but they haven't answered to anybody else in a very, very long time.
Quasar once fought a being called Skeletron, last survivor of an ancient race of robots called the Tugentine Techenium; he claimed that his race once tyrannized a huge chunk of Marvel space, but the organic races of that era united to destroy them.
One specimen appears in one issue of Paperinik New Adventures. The comic doesn't give an explanation (only theories) about how such lifeforms and subsequent civilizations started... but one about how it ended.
Robots: The robots from the computer-animated movie. They've formed their own society, and humans are nowhere to be seen or mentioned.
*batteries not included: The little robots can eat scrap metal and use it to build accessories for themselves, or even baby robots.
Even all the animals in their world are also vehicles: we have farm and construction equipment standing in for cattle, tiny VW Beetles for insects, toy cars for dogs/cats/rodents, remote control aircraft for birds, and model trains for snakes.
The Iron Giant. He can heal himself and he eats metal to live. His "stomach" even starts growling if he goes too long without food.
In 9, the sackdolls are mechanical lifeforms imbued with the spirit of their creator.
As mentioned in the Western Animation folder, the Transformers are described as "autonomous robotic organisms." The term "Autobot" is apparently an abbreviation of this term.
The films series plays heavily with the "lifeform" part as they eat, bleed, urinate and reproduce like any other lifeform despite being mechanical. During a fight, Optimus Prime even spits out a tooth!
This trope becomes something of a plot point in Transformers: Dark Of The Moon. Sentinel Prime's disdain for humanity partly stems from how the humans (particularly the US Government) view the Cybertronians, especially the Primes, as just machines. As he is quick to remind Optimus, "On Cybertron we [the Primes] were GODS!"
Code Of The Lifemaker has a whole robot ecosystem. An autonomous alien mining colony Goes Horribly Wrong, developing into an elaborate ecosystem on Saturn's moon Titan. There are power-generator trees, mechanical animals up to and including intelligent, civilized forms (humans call them Taloids, they call themselves "robeings") and factories as "farms" and birthing places, as well as electricity-based food. Being on Titan, there are hydrocarbon seas and an assortment of organic compounds, which the Taloids/robeings use to make tools and vehicles. They also have a form of civilization, with remarkable resemblances to late medieval Europe and particularly late medieval Italy (a Catholic-like church, feuding city states, a scientist ostracized for suggesting that the world is round, and one state with a ruler famous for supporting the arts and sciences) that has recently undergone a scientific revolution, invented the gun, and is about to get hit with a major religious upheaval on account of First Contact.
The Gaijin from Stephen Baxter's Manifold: Space. Extra points for being a naturally-evolved species. Characters first wonder who built them, but later travel to their home-planet (named 0-0-0-0) and observe an organism that was probably to the aliens what a chimp is to us—a metal crab.
Terry Bisson's They're Made of Meat is a short story which is entirely dialogue by aliens discussing their latest discovery: The messages they've recently encountered have originated from a planet which they're currently investigating, and it appears to originate from, well, meat. The creatures are made of meat. Not creatures that are part meat, not creatures that go through a meat phase, but creatures who live their entire (horribly short) lives in a stage of complete and utter meat. What's more, the messages are made by - you know how you can make a sound by hitting one piece of meat against another? They send out whole messages made entirely of meat-flapping sounds. Ghastly. They're talking about humans.
Arthur C. Clarke's short story Crusade is very similar, but the machines go further in their disgust and decide to wipe out the meat-creatures. Here they're explicitly rather than implicitly machines, and the exception that decides to destroy the rule.
The Young Wizards series has the mobiles, computer wizards (that is, computers which are wizards) whose bodies are made almost entirely from silicon. There's traces of other elements too, which are apparently necessary for the forming and destruction of chemical bonds which give the mobiles energy.
Said Wizard on Ordeal also pretty much counts as the entire race's mother/father due to this. A Power That Be actually pretty much calls her that.
In Iain M. Banks' Culture novels, the Culture rates any lifeform, biological or machine, at a given level of intelligence to be a sapient creature, including the Minds that operate ships and colonies and run the Culture itself, Drones (for whom the word "robot is inadequate) and some spacesuits. And some weapons.
Gregory Benford's Galactic Center novels include "mechs" which are implied to have evolved from self replicating von neumann machines. Left to their own devices after their biological creators destroyed themselves, errors and changes have occured in their templates over the millenia until their original functions were replaced by sentient self-direction. They seem to fear biological life to the point of genocide.
In the Animorphs books, the Chee are a race of fully sentient robots who were created by a long-extinct species of sapient dogs, the Pemalites. They hid on Earth after the Pemalites were hunted to extinction and use holograms to pass as humans; The Animorphs consider them sentient, but while whales are sentient enough that the Drode can't kill them, he can destroy the Chee because "they're robots".
Both played straight and then inverted in a novel by German SF author Walter Ernsting (a.k.a. Clark Darlton) in which a human expedition not only makes first contact with an ancient peaceful robot civilization but discovers that humanity itself evolved from an experimental line of organic androids once created by said robots.
In Peace on Earth and The Invincible by Stanisław Lem self-replicating robots did "evolved away".
Striking Steel by Lyubov and Yevgeny featured replicating "antipersonnel complexes". One side in the interplanetary war deployed them, but with generations the sum of the tolerable limits falls out of the friend-or-foe compatibility. So the whole planet was quickly stoneaged by mini-robot swarms, each blasting to crumbles anything metallic it "see" moving save close relatives, and assimilating metal that doesn't move. Survivors adapted to such circumstances and developed some... quirks. The protagonist got there alive only because his suit and parachute were radar-invisible, after his shuttle's ECM was proven not cool enough.
The Bolo Tanks are treated this way by their author, and they certainly meet the criteria, even if it has lacklustre representation from the characters in the books. One of the major themes is the disconnect between how they are treated in-universe and the fact that from the reader's objective perspective they are clearly the most honorable beings in the setting.
The unexpected occurrence of this trope is the theme of Phillip K. Dick's short story "The Second Variety".
Warhammer 40000: In the Ravenor series of books, a hive world (urban planet) is infested with robotic ravens of uncertain origin, known as The Unkindness, whose role appears to be simply to clean up rubbish from the ecosystem. However, it later transpires that a secret society knows how to control them, using them to kill their enemies and strip their bodies down to skeletons.
Gene Wolfe's The Urth of the New Sun has an interesting variant: Sidero is clearly some kind of mechanical man, but it turns out that his particular type of robot evolved out of spacesuits with built-in artificial intelligence.
The Inhibitors from Revelation Spacewere organic lifeforms that became sentient, self-replicating machines millions of years ago.
In Sergey Lukyanenko's Line of Delirium (inspired by Master Of Orion), the Meklar are Lizard Folk who have replaced most of their organs with mechanical parts, including integrated weapons (plasma cannons and stunners in the chest plates). They are also superb unarmed combatants, given their machine-like reflexes. Their leader is known as the Perfect One, presumably because he replaced as much of his body as he could. A human sect views the Meklar as the most perfect beings in the universe and attempts to cyborgify themselves.
An interesting case in Isaac Asimov's short story "Victory Unintentional", where humans send three highly-durable robots to the surface of Jupiter in order to study the Jovians (an advanced race that evolved on the planet), who wish to destroy humanity. After a series of mishaps, during which the aliens note the robots' durability, they sue for peace. The robots realize that they never told the Jovians that they were artificial lifeforms. The Jovians simply assumed that all humans are extremely tough mechanical organisms.
Early Marvel Comics character Doctor Druid once pulled this scam intentionally. He's a telepath, so he sat inside a gigantic crane with a wrecking ball and communicated telepathically with the alien invaders while slamming the wrecking ball into their ship. Fearing they would face an entire planet of such beings, they retreated.
Bill the Galactic Hero on the Planet of Robot Slaves by Harry Harrison has the titular character end up on a planet locked in a civil war between two factions of mechanical lifeforms. According to one of the leaders, they have naturally evolved on the planet from primordial oil pools, although this doesn't stop them from building new ones.
Battlestar Galactica: The Cylons (which stands for CYbernetic Lifeform Node), especially in the new series where they are going organic. A bit different to some of the other entries as they make the transition from machine to living race within the timeframe of the setting. Interestingly no one on the Battlestar Galactica, no matter how much they hate the crazy toasters, ever seems to question the fact that they are alive and sentient.
There was a lot of questioning this early in the series that went on all the way to the second season, with statements like "you have programming, not a soul", or "you can't rape a machine". Only after the occupation of New Caprica the general populace has had enough experience of the Cylons to realize that they are really people, even though machine people mostly considered somewhat unfairly, but for a fairly good reason (almost destroying the human civilization), evil.
What's more, the Cylons can reproduce biologically. The current generation have only done so once and not amongst themselves - rather producing a Half-Human Hybrid - but their forebears, the Thirteenth Tribe, reproduced amongst their own people all the time - resulting in a self-sustaining purely Cylon population: Cylon kids, Cylon grannies, Cylon aunts, etc. - until they built their own robots and got almost wiped out.
The Original Series Cylons were Lizard Folk who turned themselves into robots. Unlike most who do this, they still act just like any normal race, only they happen to be robots.
In the Expanded Universe novels, they go on to have a civil war between the all-mechanical Cylons and the partly-organic Cylons. This gets them off the Colonials' backs for a while, but they fear that whichever side wins will be that much stronger when the war ends and they turn their attention back to humanity.
The Power Animals of Hyakujuu Sentai Gaoranger are (not entirely) mechanical lifeform nature spirits. Their bodies are formed from the Earth itself. They just look like robots because they use metals for their skins.
Seijuu Sentai Gingaman. And the Power Rangers counterparts of Gingaman and Gaoranger, Lost Galaxy and Wild Force, keep these qualities (Zyuranger's counterpart, the first season of Mighty Morphin' Power Rangers, on the other hand, turned the gods into "magical" giant robots with some degree of sentience - how much was never really explored, though they act on their own at times, and one episode had the Sabertooth Tiger Zord showing Trini the way to something needed to break the Monster of the Week's spell. Just one more area where MMPR kept the line between the tech and magic components of the same powers and equipment vague.)
The Machine People in the Rifts Phase World setting.
Alternity: The mechalus from TSR's short lived game are an alien race that merged with machines at some point in their history. Essentially, each mechalus is born as a cyborg, pre-implanted with Nanotechnology from its family line. They were later adapted for the D 20 Modern book d20 Future, under the name "aleerin".
Eberron's warforged were mass produced by humans as pseudo-living soldiers for the Last War, and when the war ended they suddenly found themselves without a purpose. Which would be fine if they weren't sentient beings with the same emotional range as their creators. By the time the setting opens warforged are facing Fantastic Racism while trying to either integrate into humanoid society or create their own.
Bizarrely while they don't need to eat, drink or sleep as expected 4e versions are still vulnerable to disease, poison and mind effecting attacks.
Dungeons & Dragons: The modrons, clockwork beings from Mechanus, the Plane of Law in the Planescape setting. It's debatable whether they're actually mechanical, though, since they're basically the spiritual embodiment of Law in the same sense that angels represent Good and demons Evil. Supplanted by the Inevitables for third edition, who are more clearly robotic in nature (visible gears and whatnot).
KULT has the symbiotic lifeforms called Techrones. Aside from that, any mechanic equipment can be a vile lifeform in disguise.
Subverted in Warhammer40k — Necrons are undead robots, having turned to such in their quest for immortality.
Robots in Sim Earth, which can be obtained by nuking a nanotech city.
An interesting example is the Mega Man universe. What started as ordinary robots in the original series were replaced by the ridiculously human Reploids in the Mega Man X series. After many events and a brush with The End of the World as We Know It, they finally receive equal standing with humans in Mega Man ZX. A few millenia pass, and both humans and sapient robots had pretty much become one species by the time of their extinction in the Legends backstory.
The series also demonstrates some very odd background art. In X, for example, humanity has seen fit to construct robotic woodpeckers that pluck robotic worms from robotic trees on robotic cliffs. Yeah.
The dragons of the Wild ARMs series are a race of living Transforming Mecha. The demons, which have bodies made of metal, blood made of mercury, and astral forms made of electric signals, also count.
The Mmrnmhrm race of Star Control. Except they can't reproduce without the Mother-Ark, which has apparently not made any more after the initial batch (which was in the millions).
Many Steel-class Pokemon belong to this trope. (Magnemite and evo's, for example)
In Ōkami, this is mixed with Science Is Bad in that Yami, the God of Darkness is the source of all technology and as such appears as a giant mechanical orb. This also applies to Lechku and Nechku, demonic Owls made of Clockwork, and several other demons.
Other examples in the series include the most powerful Angels, such as Metatron, Sandalphon, Melchizedek and Ophanim, and the Innocents from IMAGINE.
The bosses in the Darius series of side-scrolling shooters appear to be enormous spaceships in the shape of fish (or occasionally other aquatic life). According to the official backstory, however, these "huge battleships" are actually themselves alive.
An odd borderline example presents itself in the S'pht race of Marathon, which are apparently non-sentient creatures bonded with a special Black Box upon birth which grants them sentience (courtesy of the legendary Jjaro). They think absolutely nothing of it until the first time they examine a normal sentient lifeform, they are completely flabbergasted and horrified by the very idea that their "birthing operation" may be what grants them sentience.
In the Galactic Civilizations series of games, the Yor were originally created by the Iconians as servants but revolted against them, nearly wiping them out and forcing them from their homeworld.
The Meklar from the Master Of Orion series probably count. Although they originally started out as cyborgs, the Meklar race gradually became more and more robotlike to the point that it ultimately split in two in the third installment. Those who ultimately became purely mechanical kept the Meklar name, while their still partially-organic counterparts became the Cynoids.
The geth. It is in fact common knowledge that they were created by the quarians, an alien race which is not extinct or long-disappeared, but their quick development from questioning if they have souls to mechanical lifeform species raises some taut ethical conversations.
The Reapers are an even more straightforward example, although they get testy if you ask who "built" them. Mass Effect 2 reveals that they're at least partially organic. They reproduce by rounding up thousands of people, liquefying them, and pumping the genetic paste into a shell.
Sovereign: Organic life is nothing more than a genetic mutation, an accident.
While Spore leaves their exact origins a mystery, this, and their incredibly violent tendencies, are the two things quickest noted about The Grox.
Most of the Titan (who were themselves made out of metal) constructs fall into this category, including the Earthens (who would become the Dwarves as a result of the Curse of Flesh) and the Vrykul (who would also become flesh because of th Curse and are the ancestors to the humans).
Barring the Mechagnomes, most of the Titan creations appear to have started as living or magically animated sculptures rather than machines. Some of the apparently uncursed giants can actually be mined like mineral nodes as opposed to dropping parts.
Smithy and the Smithy Gang in general in Super Mario RPG are this, living evil weapons in particular...
Three planets in Meteos are inhabited by mechanical life forms (or at least deemed as such): Machines on the planet Grannest/Smogor, built to serve the original inhabitants who have left without them; the robots on Mekks, who are fully sentient and run a space mine; and the electrical beings on Wiral/Neuralis, who tend to the surface while the actual population lives inside their world. Some of the Wiralons/Neuralisians are made of electricity and can also be considered as Energy Beings.
Platformer B.O.B. is a story of an adolescent robot driving his robotic parent's car to pickup his robotic girlfriend, complete with "where were you" remarks.
Flash game Alchemia involved life forms created by an elixir of life. This results in a mechanical life form that became a ghost when it crashes and need to inhibit a new metal body.
The Minirobots from Mini Robot Wars, who are created by the artificial planet they live on.
The Xenon of the X-Universe series began life as artificially intelligent terraforming drones created by Earth. A faulty software update made them go rogue and eventually they became fully sapient and "the greatest threat to biological life that ever existed throughout the whole universe." They have their own shipyards to build more of themselves. Predating them by roughly 500 million years are the Sohnen, a robot species used by the Ancients as an intermediary to the young races.
The Mechon from Xenobladeare NOT this, much to the surprise of the protagonists. The real mechanical lifeforms are the Machina race, inhabitants of Mechonis. The mechon are just the creation of one of them.
Basically everyone in Primordia is a robot of one sort or another, what with humans having gone extinct.
The Armada from Pirate101 consist of clockwork soldiers that were built to help Valencia win the Polarian War. They were highly successful but have since taken over Valencia and have been threatening to take over the entire Spiral. They to not tire, they do not give up, they do not die!*
But they break if you hit them hard enough.
The Machine Men from Rice Boy actually grow as they age. One of them nearly dies from poisoning.
Some of the backstory provided in "Order of Tales" indicates that they even evolved from a more primitive rock-based form to their metallic, mechanical appearance in the present-day of the setting.
Homestuck has quite a few examples. One of them is Jade's dreambot, which acts as a surrogate body when she's asleep. Another is Liv Tyler the Rabbit, who is at least semi-sentient. Aradia's soul remains inside a robotic body for much of the Hivebent Arc. In addition, Dirk's Autoresponder is a computerized copy of his brain that lives inside of a pair of glasses.
The TicTocs of Gunnerkrigg Court aren't revealed to be robots until one of them gets autopsied. It then takes root in the ground and starts growing. Did we mention that, aside from making their distinctive tic-toc noise, they look like birds?
The precursors to the Court's modern robots are part this and part golem as well. Kat does what is effectively heart surgery on one.
The series of daylogs on Everything2 following Moloch36 and his days in shaft thirteen, level ninety-nine. First can be found here.
Orion's Arm features many "Mechanosystems" both Terragen and Xenosophont in origin. Including one named Stanislaw.
The artist Extvia on deviantart has this in his SYNC series, with nanotech-based anthros.
Transformers contains the most popular examples of this trope. Most versions of the race's origin even hold that they were created by Primus, either a Sufficiently Advanced Alien or an actual god depending on who you ask. Starting with Beast Wars, Transformers even have souls, called "sparks".
In the original cartoon, however, the Quintessons (themselves either mechanical or cybernetic) built Cybertron as a factory to produce robot slaves (non-transforming proto-Transformers). The robots rebelled, and kicked the Quintessons off Cybertron. Millions of years later, the Transformers have forgotten all about them, but the Quintessons still want their planet back.
In the comics, their original backstory was that they evolved from naturally occurring gears, levers and pulleys. Uh... yeeeeeah. This was eventually quietly forgotten in favor of the Primus origin, in which the Transformers were to be his trump card against his Evil Counterpart, Unicron.
Oddly, this is superficially similar to an origin provided in an obscure text story from a Japanese magazine. Basically, Cybertron was once a space station that developed sentience, absorbed materials from throughout the galaxy, and eventually changed itself into a planet with robot inhabitants.
Transformers Prime goes into depth about their nature, when the Autobots meet the main human cast for the first time, Raf asks "So, if you guys are robots, who made you?" Ratchet is actually insulted by the implication that they were manufactured.
A humorous moment in "Masters and Students" has Jack trying to work on a regular motorcycle and Arcee (a motorcycle Transformer) is referring to parts as "doohickey." As Jack points out the irony that she doesn't know how a motorcycle works she asked if he could replicate a small intestine.
"Operation Bumblebee" has Starscream flat out state that what they have is more biology than machinery, as when Bumblebee gets his T-Cog stolen (Transformation Cog) it is stated that it is basically an organ and Ratchet can't just make a replacement from junkyard scrap. Given that some parts can be replaced, though, (Starscream gets a new arm at one point) it's probably akin to human prothetics where things like limbs can get replacements but internal organs are a lot harder to do.
Another episode of the original series involved a Decepticon and an Autobot ending up with each others transformation cogs. They had to be precisely adjusted in order to work properly (having only allowed unstable, partial transformation before the adjustments), which conforms to the idea that they function similarly to organs.
Things like surface plating appear to be fairly easily replicated and replaced. The "cosmic rust" incident began with Megatron having his chest plating damaged by a high speed projectile and Starscream telling him after they got back to base that a replacement would be fabricated "when they get around to it".
Though the mechanical characters in Thomas the Tank Engine know that they are artificial and even talk about being built by humans, they still fit this definition:
The engines mention, often to other motorized characters, like cars or helicopters, that they don't want to be anything other than locomotives.
There are a few sets of 'twin' engines that are canonically referred to as brothers.
When the issue crops up, the steam-powered and diesel-powered engines appear to be considered two separate races.
The Animal Mechanicals world is entirely populated by these, including the titular Animals, all designed to look like kid's building blocks.
Futurama: Professor Farnsworth unwittingly creates some. The life-forms evolve so fast that, within a few days, they put him on trial for promoting creationism.
Scientists speculate that Mechanical Lifeforms are the most likely form of alien "life" we'd encounter. There's even a term for them: Von Neumann probes. Self-replicating, spacefaring machines, they may have been created to colonize and terraform other worlds for their creators, or to communicate with sentient life and/or guide their evolution (think The Monolith), or even designed by Absolute Xenophobe aliens to seek out and kill all other forms of sentient life. TL;DR version: The sentient aliens most likely to exist would either be Autobots or Decepticons, or the creations of the Anti-Spirals. Another thought is that an alien species with nanotechnology might propagate itself throughout the galaxy (and potentially beyond) through Von Neumann probes that rebuilt their species from biological materials found on-site. In other words, alien nanites drift through space, arrive at your world, there's a massive gray goo catastrophe, and a few centuries or millennia later... Ker-poof, their whole society and biosphere has been rebuilt on another planet. Any original intelligent inhabitants of the planet (read: you and me) are gone along with the rest of the biosphere, but hey, you gotta break a few eggs to make an omlet.