Science fiction is the fiction of the future -- almost exclusively the future of humanity. It almost always shows descendants of today's human population in futuristic settings, doing things that few even consider possible today. Much of it shows ONLY humans. A few stories include non-humans, ranging from semi-intelligent to super-intelligent aliens, allied with, indifferent to, or in direct conflict with humanity. Sometimes aliens are the bad guys, sometimes the good guys, and frequently mostly for scenery or a plot device.
How realistic are these depictions? We'll never know if/until we meet them. The one thing that's certain, however, is that IF you're writing Science Fiction, and you use alien creatures in your work, how realistic are your characters to your readers?
Past depiction of aliens run from human or almost-human to so bizare they're almost impossible to relate to. Some of them work, some of them don't. Let's look at what a realistic alien needs to possess in order to "work" -- to appear realistic to readers.
One of the major considerations for creating aliens is to construct a realistic social structure. Individuals, regardless of the type of creature you create, cannot extend beyond it's own ability, and no individual can do everything it takes to build civilizations. To expand beyond the individual requires a social structure, whether it's the family, tribe, nation-state, planet, or greater. The more complex the alien civilization, the more complex the social structure needs to be. This doesn't mean you have to create that social structure in your book, but you MUST create it in your mind, and remember it when writing your words. This is THE major blunder writers make when working with aliens.
Social structures need advances in individual capability to develop and mature. That means language, probably art, and definitely science. It doesn't mean it has to be anything LIKE human language, art, and science, but that the alien society has developed something similar.
No alien is going to get beyond the early hunter/gatherer stage without the ability to create and use tools. While a multiple-armed tentacled creature could develop the ability to use tools, its society would be definitely more constrained than something like humans with opposable thumbs (it takes a tremendously greater amount of mental activity to control twelve limbs than it does to control two limbs, two hands, and ten fingers. Intelligence isn't likely to expand beyond basic tool usage, because too much brain activity is being concentrated in manipulating appendages -- or otherwise brains are so huge they don't fit in confined spaces).
Our alien has to have sense organs in order to percieve its environment. Another strike against our tentacled creature above would be the probable necessity of having a unique sense organ for each of its manipulative appendages. Coordinating that much information would again require such a huge amount of brain space that there would be little room left for intellectual pursuits.
That doesn't necessarily mean that our alien's sense organs need to be the same ones we use. There may be an alternative to the five senses we employ (sight, touch, hearing, taste, etc.). There may be a telepathic sense that provides all the external evaluation sensors we employ, or an all-inclusive sense of touch that lets the individual interact with its world. It isn't as important what they are as that they exist, and have understandable parameters that establish what they can -- and cannot -- rely upon their sensors to provide.
Those sensors also have to relate directly to the world where they developed. For instance, let's say that our aliens developed on the planet around a red dwarf. Red dwarfs don't give off enough light for the human eye to function well. The alien's eye would have to compensate for that dimness in some way -- by a better vision transfer means than our rods and cones, of larger eyes/vision organs, or by the ability to significantly manipulate the vision organ to provide a greater range of operating, such as down into extremely low frequencies. At the same time, a creature that has developed around an F-type primary needs the ability to restrict the amount of light that enters the sense organ, or they'll end up with eyestrain.
We need to back up just a minute, and inject a little hard science. When we talk of red dwarfs and F-type stars, we're talking about stars of different brightness -- and also, of different evolutionary development rates. The best source of information for this is the Hertzsprung-Russell (Hertzberg-Russell) diagram (do a Google search). ANYONE who wants to write science fiction and wants to get out of the Earth's solar system should know this information implicitly.
The Hertzsprung-Russell diagram plots stars according to their size/surface temperature. There are three basic groups of stars: the MAIN SEQUENCE, the WHITE DWARFS, and the GIANTS. The main sequence stars are divided into CLASSES based on their surface temperature, from the hottest to the coolest. These solar classes are O, B, A, F, G, K, and M, with a half-dozen other classes to classify the non-main-sequence stars. An easy way to remember this is the mnemonic "Oh Be A Fine Girl, Kiss Me." Stellar main sequence evolution also relates to size/surface temperature, with the largest stars being the shortest-lived, while the coolest stars live the longest. This is because of the star's energy budget: an O-class star must burn massively more hydrogen than an F-class star, and an F-class star burns significantly more hydrogen than an M-class dwarf.
All of this has meaning for your alien civilization. An O-class super-giant would never exist long enough for a civilization to form before exploding into a supernova, destroying any planets that might exist around it, and any developing civilization that might have existed. On the other hand, an M-class dwarf would probably not provide enough energy for something akin to plants to develop, much less a civilized society.
This doesn't mean that M-class dwarfs don't have planets -- we've already discovered some that do. They just aren't a very good choice to find an advanced civilization. The best, most logical choice would be around the suns that can provide enough energy for a civilization to come into existance, enough longevity for the long, slow evolutionary process to take place, and sufficient energy to keep the process moving. This pretty much leaves with F-, G-, and K-class stars. Luckily for us, these groups make up the second-largest number of stars in our galaxy (and any other, for that matter), behind M-class dwarfs.
All of this leads us to our next biggest area of consideration: the environment in which your civilization develops. That depends on two things: the output of the source of energy, and the distance from that source where your planets are located -- the so-called "inhabitable zone".
What this actually defines is the zone where water can exist as a solid, liquid, and gas, at least during part of its stellar cycle. This more or less defines the "inhabitable zone" for humans and aliens similar to them. That DOESN'T mean that some weird form of life can't exist in other areas, such as the clouds of Jupiter-type planets, the surface of Titan and similar environments, or other areas, just that it's less likely. The energy budget for such creatures would be so small their movements would have to be measured over decades, or the source of energy would have to come from other sources, such as gravitational forces that heat Jupiter's upper atmosphere to temperatures similar to those found from central Europe to the tropics (Robert L. Forward's novel "Saturn Rukh" handles this very well, and very intelligibly).
So, our alien has to fit his environment, which has to fit his world, which has to fit his sun. That means his development through stages to intelligence, then up the ladder to where he can at least contemplate life elsewhere, to where he can physically search for that life among the different stars.
One other thing needs to be discussed, then I'll shut up: sex. How do you handle alien sex? You cannot ignore it -- just as it's a main driver of human development, it will most likely be a main driver of alien development. The drive to reproduce is what keeps the numbers climbing, and civilization being forced to come up with more and more ways to accommodate the higher numbers. Civilizations that don't have a population growth don't have any other kind of growth, and soon die.
Alien sex can be anything from asexual (very unlikely, but possible, especially on a planet with surface conditions similar to Titan) to requiring up to a dozen individuals to succeed (very possible, especially on a planet around a very active sun, where multiple copies of the genetic record are needed to keep mutations from destroying the society). Sex, like the other characteristics of your alien, are shaped by its environment, not our wishful thinking.
The following is just a personal evaluation of aliens, and only shows how I have developed and depicted the aliens I have used in my stories.
I believe that there are far more planets in our galaxy than most scientists are willing to admit. I also think that much of the identification of large, close-in gas giants is actually the detection of multiple planets, instead of one. I can't see gas giants being viable around F-type and hotter suns. The solar wind would strip any atmosphere they may have from them, leaving a rocky core which would be significantly smaller than what's being measured.
I also believe life is far more pernicious than most people think. Life is found everywhere on the planet Earth that it can exist, from the permanent snow line to the deepest depths of the oceans. I would expect life to develop wherever there was the possibility for it, which would include an energy budget to allow it to exist, the raw materials for that type of life, and the time for it to have evolved. (I also believe that when God created life, He created the OPPORTUNITY for life to develop - a viable energy budget, the raw materials, and a gentle nudge based upon the laws that govern the universe [created by God, so they're God's Laws].
With life, Intelligence should also have developed. That does NOT mean it's "our" kind of intelligence, or even something we can understand -- just that intelligence will develop anywhere there's the possibility for it. All in all, I think we have a lot of friends - and enemies - awaiting us once we learn how to get to them.