What he has done with both AFTER MAN and THE NEW DINOSAURS is so exciting that I feel it deserves a special name of its own: Alternative Zoology. -Desmond Morris, in the introduction to Dougal Dixon’s ‘The New Dinosaurs’
Alternate Zoology is a rather difficult subject to work with. It starts by saying, “How would this creature be different if it filled this niche, as well as or instead of its’ own?”
But the person working on something like this, especially if this zoology is for some alien planet, must be careful that their creatures make sense. On another planet, one must answer the questions, what does this creature eat? What eats it?
For example, look at the world of Pandora, from Avatar. The creatures of the world were designed by a team of biologists and zoologists, to fit into the ecosystem and atmospheric conditions of the world, such as lower gravity, which enables large creatures with six legs- a feature that no vertebrate on Earth has ever had.
Another example of successful Alternate Zoology is that done by Dougal Dixon, who, in two different books, predicted changes in animals if humans disappeared, and changes in dinosaurs if they had not gone extinct. The first can be found here: http://www.sivatherium.narod.ru/library/Dixon/main_en.htm
The second, here: http://www.sivatherium.narod.ru/library/Dixon_2/00_en.htm#fore
These two books are some of the best AZ I have ever seen. Take a look.