Well, if you're interested, here's what I did with the Ursaring (I started with a pokemon that's almost identical to its base creature, the grizzly bear, to make sure castes were the only thing I was testing).
[CREATURE:URSARING]
[DESCRIPTION:In forests, it is said that there are many streams and towering trees where an Ursaring gathers food. It walks through its forest collecting food every day.]
[NAME:ursaring:ursarings:ursaring]
[CHILD:1][GENERAL_CHILD_NAME:teddiursa cub:teddiursa cubs]
[CREATURE_TILE:'O'][COLOR:6:0:0]
[PETVALUE:300]
[PET][trAINABLE][MOUNT]
[NATURAL]
[LARGE_ROAMING][FREQUENCY:5]
[POPULATION_NUMBER:2:3]
[CLUSTER_NUMBER:1:1]
[BIOME:FOREST_TAIGA]
[BIOME:ANY_TEMPERATE_FOREST]
[BIOME:SUBTERRANEAN_CHASM]
[CURIOUSBEAST_EATER]
[CURIOUSBEAST_GUZZLER]
[GRASSTRAMPLE:0]
[LARGE_PREDATOR]
[NO_WINTER][MEANDERER]
[PREFSTRING:strength]
[PREFSTRING:fondness for honey]
[BODY:QUADRUPED:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:5TOES_FQ:3TOES_RQ:MOUTH:GENERIC_TEETH_WITH_LARGE_EYE_TEETH:RIBCAGE]
[BODYGLOSS:PAW]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[SELECT_TISSUE:HAIR]
[INSULATION:200]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:BODY_HAIR_TISSUE_LAYERS:HAIR]
[USE_MATERIAL_TEMPLATE:NAIL:NAIL_TEMPLATE]
[USE_TISSUE_TEMPLATE:NAIL:NAIL_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:TOE:NAIL:FRONT]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SKIN:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDONS:LOCAL_CREATURE_MAT:SINEW:200]
[liGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[HAS_NERVES]
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[GETS_WOUND_INFECTIONS]
[GETS_INFECTIONS_FROM_ROT]
[USE_MATERIAL_TEMPLATE:PUS:PUS_TEMPLATE]
[PUS:LOCAL_CREATURE_MAT:PUS:LIQUID]
[BODY_SIZE:0:0:8800]
[BODY_SIZE:1:0:60000]
[BODY_SIZE:2:0:125800]
[BODY_APPEARANCE_MODIFIER:LENGTH:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[MAXAGE:20:30]
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_CATEGORY:HEAD:BY_CATEGORY:TOOTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[ATTACK:SCRATCH:CHILD_TISSUE_LAYER_GROUP:BY_TYPE:STANCE:BY_CATEGORY:ALL:NAIL]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
[DIURNAL]
[HOMEOTHERM:10067]
[SWIMS_INNATE][SWIM_SPEED:2500]
[MUNDANE]
[CASTE:URSARING_FEMALE]
[FEMALE]
[CASTE_NAME:ursaring:ursarings:ursaring]
[CASTE:URSARING_MALE]
[MALE]
[CASTE_NAME:ursaring:ursarings:ursaring]
[CASTE:TEDDIURSA_FEMALE]
[FEMALE]
[CASTE_NAME:teddiursa:teddiursas:teddiursa]
[CASTE:TEDDIURSA_MALE]
[MALE]
[CASTE_NAME:teddiursa:teddiursas:teddiursa]
[SELECT_CASTE:ALL]
[SET_TL_GROUP:BY_CATEGORY:ALL:HAIR]
[TL_COLOR_MODIFIER:BROWN:10:BURNT_SIENNA:10:MOSS_GREEN:1:TAN:10:OLIVE:1]
[TLCM_NOUN:hair:SINGULAR]
[SET_TL_GROUP:BY_CATEGORY:ALL:SKIN]
[TL_COLOR_MODIFIER:BROWN:1:BURNT_UMBER:1:CINNAMON:1:COPPER:1:DARK_BROWN:1:DARK_PEACH:1:DARK_TAN:1:ECRU:1:PALE_BROWN:1:PALE_CHESTNUT:1:PALE_PINK:1:PEACH:1:PINK:1:RAW_UMBER:1:SEPIA:1:TAN:1:TAUPE_PALE:1:TAUPE_SANDY:1]
[TLCM_NOUN:skin:SINGULAR]
[SET_TL_GROUP:BY_CATEGORY:EYE:EYE]
[TL_COLOR_MODIFIER:IRIS_EYE_BLACK:1:IRIS_EYE_BLUE:1]
[TLCM_NOUN:eyes:PLURAL]
[SELECT_MATERIAL:ALL]
[MULTIPLY_VALUE:2]
[SELECT_CASTE:TEDDIURSA_FEMALE]
[SELECT_ADDITIONAL_CASTE:TEDDIURSA_MALE]
[BODY_SIZE:0:0:1000]
[BODY_SIZE:1:0:5000]
[BODY_SIZE:2:0:8800]
All creatures in the basic raws seem to be divided into male and female castes, so that's what I did here too, though I'm not sure whether it's necessary.
The way I've been doing it, all members of the species are called by the name of the base evolution as children ("teddiursa cub") even if that's not what they technically are. This will be a problem with things that radically change as they evolve (flying charmander hatchlings would be a problem for example) so they might have to be born fully evolved.
This probably
does cause the problem that they all show up as "ursaring" in the embark screen and the diplomat's request window. I personally think that's a reasonable compromise, but some people might disagree. I tested them in the arena, though, and was able to tell ursaring from teddiursa.