Humanoid

A humanoid is a creature that bears a superficial resemblance to a Human. Humanoids, while not related genetically, evolved some traits convergent to that of humans. The key features include the following:
 * Large cranium
 * Two arms which contain digits and an opposable thumb
 * Two legs which allow bipedal movement
 * Upright stance
 * Neoteny

Plausibility
Because we have yet to discover any other sapient beings in the universe, we cannot be certain if a humanoid appearance is extremely common or very rare. However, the humanoid body shape has many evolutionary advantages that differently shaped creatures lack. For example, arms and fingers allow for the manipulation and construction of tools. Having a large cranium allows more space for a more complex brain, therefore allowing the creature to make decisions and even be self aware.

We can be sure, however, that it is extremely unlikely that other humanoids in the universe will look exactly like humans except with forehead ridges. More realistically, even though they possess a similar body shape, internal structure and outer appearance may be completely different. When you design a new sapient species, keep this in mind. While you may use the humanoid body plan, make the outer appearance unique by listing its various adaptions.

As a writing device
Although the humanoid body plan is arguably unoriginal, there is a good writing reason to do so. It may sound egotistical, but it has been scientifically proven that humans (the target audience) respond better to creatures that look similar to themselves. This is the reason why humans enjoy cats and dogs more than snakes and insects.

This is also the scientific reason for why humans find certain things cute. Humans are genetically predisposed to parent and care for anything that happens to resemble a human infant. As a result, ignoring the needs of a child or infant is almost universally considered cruel or inhumane. This doesn't only affect humans; it even affects other animals. Some predators will deliberately leave an infant prey animal alive and even care for it rather than eating it. To put it simply, cuteness is a form of mind control.

If you want to write a story that heavily focuses on emotion or internal conflict, making your character a humanoid allows you to have a character that is easily relatable to the audience. If you choose to write a story about children or infants, keep the above paragraph in mind as you write.