Convergent evolution can throw up some remarkable similarities- like between fish, plesiosaurs, cetaceans and phocids for example- but that is because of the similarity of the environment- the physics of swimming haven't changed in half a billion years. As far as we know (working from a sample of one) there's no specific evolutionary pressure that engineers intelligent beings to be of a particular shape. So it would be an astounding coincidence if extra- terrestrial intelligent beings were humanoid in form.

Let's try to "design" an alien from scratch. Anything that has to face up to gravity (how much gravity?) needs a supporting scaffolding. Ours is internal and built like a sailing ship's rigging (spars tensioned by muscular "ropes"), arthropods have external tubular supports. Those are the only two designs that have made it to any size out of water, but there could be others- an internal or external space-frame, for example.

The segmented form shared by almost all animals big enough to see is not a given, but is an easy thing for evolution to work on- grow in complexity by adding segments, use specialised modified segments for particular purposes- so it's a probable form for an alien. And similarly bilateral symmetry is likely though not certain.

The alien will need sensors of various sorts. The sensible place to put at least some of them is at the front- you want to sense what's coming- and the sensible place for the processing unit is near them. And it's precious, and represents a huge and probably ireplaceable investment, and so needs to be protected. So our alien will have a hard head at the forward- going end.

Eyes- photosensors- have evolved in many patterns, and they are an obvious evolutionary move, so the alien will have them. The frequency response will be designed for the planet where they originated, but there is an astounding choice of possible structures, from insect- style compound eyes, through pinhole cameras, to several different sorts of lensed designs. And although vertebrates normally have the two that we inherited from fish, that's just contingent. Aliens will probably have a minimum of two, but there could be several more.

They will have in some degree most of the other senses we have, as information is evolutionarily useful. They might not have all of ours, just as some of our senses are vestigial compared to other animals. And they might have others- electrostatic or magnetic sensors.

Locomotion is necessary. Legs are better than wheels, as they work on more surfaces. There needn't be only two, and one or none is a possibility- but an intellient being will need manipulators as intelligence evolves out of curiosity and the ability to alter the world. And arms are an obvious form. They needn't be modified legs- they could be modified mouthparts for example, and elephants have trunks.

Communications- we use sound, air pressure variations- most of us, that is. Deaf people use manual signs, and that seems to work very well. An alien could also use, for example, colour changing (their colour) light patches to talk to each other. Or have a magnetic generator that works with their magnetic sensor- we would think they communicated by telepathy.

They would probably reproduce sexually- this has great, though not absolutely overwhelming, evolutionary advantages- see bdelloid rotifers. Or they might have a different way of sharing their genetic information (which probably wouldn't be DNA). They might even do it like kids used to swap cigarette cards.

So here's my alien- a tube with an internal space-frame a bit like a sponge but jointed for flexibility, with a domelike head at the front end with two flexible trunks and six lensed eyes. They see in the near infra- red. It has six legs and two arms with graspers. They appear to communicate telepathically.