How would procedural hybrid generation handle mules, for example? Which are already in the game, although as far as I know they are magically generated from nowhere, as there's currently no mechanism for hybridization. When Toady implements such a mechanism mules ought to keep the same characteristics they have now, and continue to be called mules instead of some generated name based on horses or donkeys. I have nothing against the idea of procedurally generated hybrids, but I think that some system based directly on creature definitions in the raws is needed, both to cover real hybrids with established names and traits, and for greater modding flexibility.
The hybrid system ought to account for both inclination and ability. One approach might use an ATTRACTION_CHANCE tag in the definitions for each potential parent and a HYBRID tag in the mother's definition.
ATTRACTION_CHANCE:CREATURE:CASTE:probability:probability with preference
-CREATURE, as one would expect, specifies the type of creature that is a possible subject of attraction
-CASTE would usually be ALL, but could specify a particular caste in some cases
-probability would be a percentage, determining the basic chance that an individual would find the subject creature potentially attractive
-probability with preference is a modified chance for individuals who have a preference for the subject creature. Someone who "likes goblins for their terrifying features" is more likely to find their glowing red eyes interesting and exotic, rather than horrifying.
-ATTRACTION_CHANCE would respect orientation. Potential romance would require both mutual attraction to the subject creature and compatible orientations.
HYBRID:FATHER:OUTCOME:ratio
-FATHER would be a creature ID, with or without caste specified
-OUTCOME could be a creature ID or NONE, to allow reduced fertility in hybridization. If Toady also implemented a procedural hybrid system, OUTCOME could accept GENERATED.
-ratio would be a number determining which outcome occurs at any potential pregnancy, functioning like the POP_RATIO token for castes.
For a more complex ≡system≡, see
JT's Token Wishlist. Having to write up a creature entry for every hybrid would certainly mean more work for modders, especially in mods with a lot of intelligent creatures added, but it might be easier for Toady, and I would certainly prefer to have the option of defining specific hybrids that I want to be possible, with specific names and characteristics.