I'm all about the procedural and random, and as many parts as possible to make creatures unique. My idea is simple though. For each biom there would be a slight procedural bias towards ceraint parts, colours, creature types. Some parts would be reserved for bioms, but a lot would be open to other bioms just more or less common. It wouldn't mean that all forests would have similiar creatures, just that forest planets would have more foresty creatures from time to time. Examples: Snow biom - Furry creatures and white creatures could be more common. High gravity - Small, low creatures, not jumping creatures could be more common. Low gravity - Big forms, light gliding creatures. Lava - Rocky, firy, dark creatures.