I think I suggested this a while back (or did I?).
Basically, I've found out from Toady that a lot of the entity code right now depends on there being ONE creature definition for ONE entity. Then I thought about the thing I've been trying to promote - referencing - and concluded that indeed, one creature per entity is very convenient, and at the same time it is possible to have a creature that consists of several different creatures - it will change its parameters and setup when it transitions between age categories, or depending on whether it is male of female, all using existing tags. Though the tags themselves would have to do a lot more.
Basically, instead of saying [MALE] so that the creature is recognized as male, you say [MALE:CREATURENAME_MALE], referencing a creature with the [CREATURENAME_MALE] ID for all male instances.
Furthermore, the male and female creature variants will have age categories - [CHILD:CREATURENAME_MALE_CHILD:12], for example. This will allow a child to actually be of smaller size than the parent (currently, it seems the "child" or "baby" refers mostly to the mental state of the subject, rather than the physical state), or posess intrinsics that would make it vastly different - as in the case with eggs. The number in the back means the same that it does now - the age at which the creature converts into the next phase. The details of the transition have also been discussed elsewhere, but at least pertaining to wounds and such, the method is "same tag - same state", any part on the old body with a token matching the new body's part will retain the damage and state on transition, missing parts will be lost, new parts will be unharmed.