For the purpose of creating an alien creature for use in a story one more factor must be added;

8. Communication

To begin building an alien you must consider all these factors.

1. Movement - Fly, Swim, Walk on 2 legs or more, Swing from tree to tree, Slither or "walk" in some way not involving legs such as a snake or snail. Some creatures may be able to move in more than one of these ways, but one will always take preference.

2. Respiration - This is defined as the chemical process to get energy from sugar either aerobically or anaerobically. In anything other than bacteria this would be aerobically - i.e with breathing. Oxygen does not necessarily have to be the gas required. Breathing is done with one of three apparatus, lungs for air breathers, gills for liquid breathers, and through the skin for creatures with a large surface area to body mass ratio, like insects.

3. Sensitivity - An awareness of environment or reactions to events.

4. Growth - What stages of growth does the life form go through, and at what stage is it in the course of the story,  Youth, Maturity or Old Age.

5. Reproduction - Without reproduction there is no way for evolution to take place. Reproduction can be Sexual or Asexual. 

6. Excretion - As well as simply expelling waste matter and gas also forms part of the food chain with other creatures being dependent on the excretion of others. Also can be used as means of self defense.

7. Nutrition - Predator or prey, what does the diet consist of ?, is the creature carnivore, omnivore, herbivore, does it depend on one source of food or many ?

8. Communication - Oral, change in physical appearance, release of substances, telepathy, written, or by means of an instrument (drums, smoke signals, etc).