Creating believable aliens for my scifi stories is a fascinating business for me. First of all I like to make them believable, so I have to put in a lot of effort to see what, among carbon or silicone based life, is possible. The limits are, according to some biologists and experts, pretty wide. So the next step is to try and work out how the human element in the story is likely to interact with them. That does impose some limits, especially if you want them to be able to communicate or even work together. As an early Star Trek episode demonstrated, interaction between humans and a giant slug-like creature is next to impossible.
Creatures with tentacles for limbs also tend to need a liquid in which to be really effective and efficient in their movements, so putting them on terra firma can be a bit limiting. Insectoid creatures, because of their breathing arangements, tend to need extremely oxygen rich (or even methane rich) atmospheres in order to support their size. An oxygen level much above 22% would be poisonous to human life and most mammals, so there is a restriction on exposure for the human characters in that situation. Gilled creatures need a liquid atmosphere in which to live and breath - again a bit of a no-no for humans.
Then comes the ability to communicate. Humans have vocal cords, as do some other primates, though not all, and some of the mammals as well. Insects and most reptiles don't. Surprisingly, a lot of birds don't either and as for marine anilmals ... So, this really does mean you have to be imaginative and sometimes a little elastic when 'creating' and alien character.
My favoured method is to look about me at the variety and diversity of life on our own planet and sometimes to delve into mythology. Once I've formed an idea of what the creature/character will look like and what physiological features it displays, I do a sketch. Sometimes a lot of sketches to work out the mechanics of the creatures limbs, body and movement. Then I write up a 'bio' of it. Does it wear clothes? How does it communicate? Does it look friendly or terrifying? How does it move? What sort of society does it create or live in? Does it follow the basic 'laws' of animal behaviour? (Summarised I'm told as - Can I eat it? Can it eat me? Can I mate with it? Or it mate with me? Is it a threat to my territory/young/food supply? Can I fight it?) Once I have this down on paper, I have to work out what will happen when it meets a human or a human meets it for the first time.
There is one other slightly limiting factor. Humans tend not to interact easily with creatures that are not at least slightly humanoid in shape and features. So when you need an alien your human characters can relate to, work with and talk to, they need to be at least vaguely human-shaped. It isn't an absolute, but it certainly helps.
Interestingly, I have had inspiration from a wide range of experiences, encounters and sources for all of this. The Lacertians (Out of Time) came into being as I watched a rather interesting lizard who had taken up residence on my hotel verandah in the Caribbean. He was fast, extremely clever at disguise and an absolute terror when hunting. The Canids (On the Run and now my latest Harry Heron story) came to mind after an encounter - at a distance - with wolves in the forest. When you live in the land which most likely inspired Terry Pratchett's Uberwald, it is easy to see the humanised version that inspires the myth of Werewolves.
Here is an example of the sort of note I make -
Canids
As the name suggests, these people resemble the Canine species superficially. The concept came from the idea of Werewolves, the "human wolf" concept. Thus I have envisioned a creature that has a wolf-like features and behaviours, but is humanoid in most physical aspects. The face and head is very wolf-like as are the 'hands' and the deep chest and some other physical attributes. The 'hands' are paw-like, having claws at the tips of the phalanges, though with the flexibility of fingers and a thumb.
The head and neck are covered by thick fur, though, like humans, this is as varied as human coloration and appearance. The neck is thick and, like a wolf, protected by a heavy mane. While they have a thick fur on the face, head, neck and shoulders, the rest of the body is hairless, hence their need for the specialised clothing they wear. The eyes are set quite wide, on either side of the short muzzle. This gives them a very wide field of vision. The eye is equipped with an extra lid (Like cats) and the ears stand above the head. The mouth, like a wolf's, is equipped with some serious teeth. Their speech consists of growls, sharp staccato sounds, whines, whimpering noises and a more gentle growl-like noise which may indicate pleasure or approval. Communication is always through a translation device they carry with them in human company and which has its origins on their home world, which is regulated by a living 'computer' created by the Sidhiche. This calls itself 'Canus' and is known to the Canids as the "Provider."
The Canids live in a highly organised and regulated society, due to the harshness of the seasons on their homeworld, most of the year in vast cities below ground. They have factories, nurseries for their young and all the trappings of a civilised society - but, being alien, it isn't exactly as we would expect if judging by human standards only. They are organised in 'packs' with a male Pack Leader and a female Pack Mother who imposes discipline and regulates breeding cycles. A Council of the Senior Pack Leaders deals with all matters of joint interest, conflicts and the sharing of labour and division of the products of their labour.
The species have evolved as a variation of the Marsupial species, with the female producing an egg when fertilised. The 'egg' is then carried in her pouch where it incubates and the incubus then suckles until it is old enough to leave the pouch. Canids develop more rapidly than human young, but live at least as long.
Canid Clothing
The Canid's wear clothing adapted to their physiognomy and suited to their climatic conditions and environment on Canes, their home world. It comprises a pair of trousers which reach half way up the chest on a male. On the females it is slightly different, with an opening flap allowing access to the incubation pouch. A 'shirt' style garment that includes long sleeves and mitten style 'gloves' at the end of the sleeve is worn with, when outdoors (or on shipboard) a jacket over it. This pulls over the head and shoulders, the head protruding through a close fitting 'collar' opening around which a hood is rolled when not in use.
The Canids wear heavy looking boots (but, as Harry and his team discovered while hiding out on Canes) the appearance is deceptive, the boots are, like the clothing, hardwearing, close and comfortably fitting, and very light and warm. The fit and appearance of the clothing it closely resembles neoprene a 'wet suit' worn by divers and surfers. Made from a material not unlike neoprene rubber, it is quite thick on the trousers and jackets and the outer surface is water and weather proof. A type of harness is worn over the jacket to carry tools, weapons and other items with a type of small back-pack and pouches on the front belts and the waist.
The Canids also use a cloak-like garment which has the property of bending and reflecting light to deceive the eye and make them 'invisible.' Originally a hunting tool, the Canids have found it useful when attempting to watch visitors to their world or to hide their activities from those they don't trust. Harry and Co use it in On the Run against their enemies.
During their stay on Canes, Harry and his team had to have this clothing adapted to allow their skin to 'breathe' and for perspiration. The Canids, like dogs, have no pores and don't perspire. One other difference - both cultural and physiological - meant the clothing for humans had to have some additional adaptions.
Once I've got an idea of the creature's appearance, behaviour and society, it becomes much easier to write about them. Nothing like having a clear picture! The saying that a "picture is worth a thousand words" is certainly true when writing about creatures of the author's imagination.