Rubber Forehead Aliens: Averted. With the irrelevant caveat mentioned under Human Aliens, anything that looks like it belongs to/is descended from a particular species almost certainly does/is. See also Starfish Aliens.
In popular mythology, this is usually conflated with first contact, or at least with the establishment of genuine communications between the species in question – which portrayal, unfortunately, is almost pure nonsense.
Interstellar civilization just isn’t that subtle.
Space is cold and dark. Interstellar life is the exact opposite. Between the EM penumbra, starship drive flares, the gravity-wave ripples of stargates in operation, and even some few modified stellar spectra, anyone within a couple of thousand light-orbits of the Periphery with any astronomical competence at all can have no doubt that there’s exotic life out there – with the only possible exception being those on the wrong side of the Shadow Veil.
If you’re actually trying to make contact, you can’t avoid giving advance notice. In the first first contact on record, the galari identified Extropy Rising – a slowship, not even a lighthugger – light-months out of their system, even before the inbound ship spotted the radio emissions of galari civilization. The deceleration burn of a modern lighthugger is easily visible from the next star over, and highly distinctive to boot; an optimized fusion torch or the double-peaked signature of a pion drive look like nothing else in space. As for starwisps – how many stars do you think there are that shine monochromatic green?
(And if the lighthugger in question is a linelayer, it’s going to leave a stargate megastructure orbiting in their outer system for them to look at for months, maybe even years, before a scoutship gets there. Conveniently engraved with instructions for use, even.)
This does have its disadvantages, triggering social unrest, cultural shifts, bursts of technological development, and the like, or on less developed worlds – the kind whose occupants may go unnoticed until your arrival – sometimes even religious movements. In the case of psychotics-in-waiting like the skrandar, it may well have converted them into the berserkers they ended as.
But if you want to explore the galaxy at all – well, what can you do? Even the Voniensa Republic, who are remarkably prissy about this sort of thing, have had to reconcile themselves to that.
– A Star Traveler’s Dictionary
Starfish Aliens: Most of them. Digisapiences, of course, have no bodies at all. The galari are sophont crystal-virus hybrids with inbuilt techlepathy and mechanical psychokinesis. The codramaju are pseudo-fungoids which can merge, exchange, and separate bodies and minds at will. The kaeth are vaguely draconic pseudosaurians with a metal-rich biology. The hydrogen-breathing sssc!haaaouú are fragile collections of membranes that dwell in the upper layers of gas giants. The myneni are crystal-based carbohydrosilicate amoeboids with built-in chemosynthetic talents. The mezuar are a network of collectively sophont purplish-blue trees. (Yes, as sessile as that implies, although the selyéva are green-blue plantimals – non-sessile photosynthetics – who probably most closely resemble walking broccoli.) The esseli have engineered themselves into brains with manipulating tentacles and customized personal auxiliary organs, and don’t even remember what they used to look like. (And the link!n-Rechesh are heading that way.) The qucequql are ammonia-metabolising octopi from a world of nitrogenous oceans. The múrast would be simple multiheaded snakes, except that they breathe methane, live in oceans of hydrocarbons, and their primary body structures are constructed of ice. The ulakha are metal-plated, fast-moving lizardoids who think Venerian conditions are just about right for a planet. The linobir resemble furless, leathery-skinned, hexapedal, hermaphrodite bears. The shan kari resemble larger versions of Terran mustelids fairly closely, actually, except they prefer to breathe warm methane. The mirilasté are legged-serpents with skin we would recognize as essentially plastic, who breathe the most astonishingly noxious fluorine-hydrocarbon soup. The ktelaki are furry arachnids with trilateral symmetry and multi-branched legs. The seb!nt!at are star-dwelling creatures of plasma and electromagnetic force. The celsesh are quadrilaterally-symmetric with a fused-barrel body plan, and sensory organs on stalks in lieu of a head. The embatil are worm/tentacle creatures whose life cycle begins with individuals, but which merge into single creatures as they mature – while transforming a ganglionic into a collegiate intelligence. The tennoa are chlorine-breathing radial-crabs blessed/cursed with obligate utilitarianism…
And that’s all before we get to uplifts, neogens, and exotic neomorphic bioshells.