The word Humanoid was first used around 1940 in science-fiction works to denote any alien species that looked vaguely human. By “human” it means bipedal, one head, two legs, an optional tail, and two arms. These are minimum requirements, on occasion there are more limbs/heads =, but vary rarely are that less.

Were these stories based on reality, one may speculate that any species, given long enough, will evolve into a form resembling a monkey on its hind-legs. This may be because an animal that is capable of walking on its hind legs will have two spare legs to use as arms, manipulators for tools. The theory would be that any animal that can use tools would become superior. Humanoids, therefore, are animals that have climbed to the top of the evolutionary ladder through the use of tools, presumably for killing other animals. The fact that they had become very efficient at killing and manufacturing might presumably mean that they had roughly human personalities, still more suited to acting than thinking.

Sentient non-humanoids would therefore have climbed the evolutionary ladder by other means. Possibly by out-smarting their fellow creatures. This would mean that rather than developing very efficient bodies, they would develop very efficient minds. Unfortunately science-fiction writers are human, and so rarely pick up on this, usually making the humanoid species the smarter one.

Of course the real reason for humanoids is that it is easier to make slight cosmetic changes to human B-movie and Star Trek/Wars actors, than it is to create a whole new look for each species.