The basics of a humanoid-based customization system would work just as well with creatures. The trick would be to figure out analogous features of each species and categorizing them together. Like, if they all have a head mesh and a body mesh of some sort, it shouldn’t be any different than setting up modular humanoids that have mesh components for head/helmet and body/armor slots.
For multiple texture layers used for markings, you could define them in a data table with a structure like: SpeciesName, BaseTexture, MarkingsTexture, EyeTexture. etc… and on construct/begin play, you’d get the data table row which matches the species name, and set texture object parameters on a dyamic material instance, plus any custom color parameters (likely stored as variables on the actor). The color part here should be no different really than how most human customization systems select hair and skin color. Though you’ll probably want to set the default color values for each species in the data table too. And depending on how different the creatures materials are, you may also need to define a different parent material in the table for the dynamic material instance to use.