I had the rare pleasant surprise one time, in finding a world in which all the creatures shared similar traits, and looked like they all were part of the same ecosystem of a planet. They all either had shells of varying shapes on their backs, or similar kinds of heads. Which inspired the following. Tag each and every monster part with two to three 'descriptors'. IE 'Multieyes' or 'Mouthless' or 'Bignose' for head parts, whether they are Small/large bipeds, small/large quadrupeds, or fish/birds. When a planet generates, it produces for itself, a randomized list of descriptor tags for all monster parts to be used. IE, Heads='Multieye', Torsos='Scalebelly', Legs='Tentalegs', Arms='Clawhands', Tail='Spiketail', and Wings='Batwings'. Upon actual monster generation, it uses only those parts that have the tags chosen by the planet's generation. In this way, the generated creatures should wind up having both variety, but also biological consistency, further creating an immersive environment.