Prime Directive= Gameplay

Generally for games a creatures primary feature or function in terms of gameplay-

as a result the need to communicate the idea or meaning of the character or creature, as a game piece, and the need to be able to see and understand what things are and what they do can take precedence over  the reality of a creature or person (if it were real). That said any fantasy elements of modifications to common sense you make should be self consistent in their fantasy logic- otherwise confusion and frustration and lack of immersion is the result.

ie attack or special attack- as a mode of transport- as a prey animal or something to collect, and the way you communicate what that feature is dictate the look of a creature.

In games you are what you do- so often the trick is conveying to the player what something does- and how that piece fits into the game or how it should work in terms of gameplay is a goal to keep in mind.

Form Follows Function

  This means the shape is determined by its function, or the the combination of parts that create the function. Its a saying or way of working  originally applied  to designing products or buildings , but it works for anything you want to design and is a good way of choosing or deciding the success of designs. It sounds a bit dry and featureless- sort of no frills, but it does incorporate decoration and styling for example if that serves the function of what it is you are designing.

Natural things and living things are almost the ultimate example of things refined for their purposes of the organism, although not exactly designed, it aplies even the most extravagant and flamboyant looking creatures  or plants.

Natural things often behave or function in a way to minimisation of energy expenditure, or make it as efficient as possible.

Seacreature-shark  fast moving/efficient-  streamlined to reduce drag- VS weedy sea dragon- doesn't have to move fast- camouflage of ornate fins looks fanciful but help it hide.