Wellcome to Coriolis! We haven't made any guidelines to create creatures from scratch. When I wrote thew B@D section I looked at the creatures I had previously created to Mutant: Year Zero and modified them to the Coriolis rules. In some cases I increased hit points to reflect a larger animal with more stamina or size etc.
I hade the luxury to have Tomas as look over all creatures and modify them so they wouldn't be overpowered.
The creatures we've written since have used the ones in the rules as a base - a shark beast could resemble the nahang, a spirit, some sort of djinni, etc.