Fantasy creatures come in all shapes and sizes. Large, small, peaceful, aggressive, animal like, or humanoid. Portraying any type of fantasy creature can be tricky. However, you can approach writing a fantasy creature the same as you would any ordinary creature.
For a humanoid, develop them the same as you would a human, then take the unique qualities of the fantasy humanoid into account to develop them further.
For a non-humanoid, you would portray them the same as you would if it weren’t a fantasy creature. Fantasy creatures often just have more complexities in what they are physically able to do, and in their looks. A fantasy creature just has different key qualities according to their species and different ways of doing things. The best thing to do, is to research some portrayals of a certain fantasy creature that you want to use. By seeing how they are portrayed in other media, allows you to see what makes that creature identifiable to the reader with common qualities, along with allowing yourself to see how you can carefully adjust the creature to fit best into your story. That’s one thing with fantasy, even the most common of fantasy creatures can be found with so many adaptations, to where you have freedom to deviate from the standard.
What would you say is the biggest key to portraying a fantasy creature well?