I have my own issues with modding creatures, but let me take a look and see what's going on.
I think the "Goomy" in [NAME] and [CASTE_NAME] have to be in all caps, but I haven't tried it otherwise.
I think you need to put a clutch size. Unless the 6th gen changed things, you still only get one egg at a time, so you might have to guess.
You need to specify what color(s) it is. The Goomy line, as you probably know, is light pink and purple with little black eyes.
You don't need the "GOOMY" after body. Just say what the body parts are. I know there are several "templates" of body type, including quadruped, humanoid, armless humanoid (used for birds) and others. You probably want
[BODY:BODY_WITH_HEAD_FLAG:HEART:GUTS:BRAIN:MOUTH]
That's taken from the worm, and it looks pretty close to what a Goomy would have.
It probably needs skin, or at least muscle. Ideally you'd want to mod in an appropriately gooey material, but for now you just need something to hold all its body parts together.
I'm not sure what, if anything, mental attributes do for a creature that isn't sapient. Mental attributes mostly affect jobs and skills.
For some reference as to what makes a "complete" creature, here's my pride and joy, the noble and fluffy aquiloup.
[CREATURE:AQUILOUP]
[DESCRIPTION:A wolf-like creature with an eagle's broad wings and razor-sharp talons instead of paws. It can be seen flying high above the virtuous temperate lands.]
[NAME:aquiloup:aquiloups:aquiloup]
[CASTE_NAME:aquiloup:aquiloups:aquiloups]
[CHILD:2][GENERAL_CHILD_NAME:aquiloup pup:aquiloup pups]
[CREATURE_TILE:'A'][COLOR:7:0:1]
[LARGE_PREDATOR]
[LARGE_ROAMING][FREQUENCY:50]
[BIOME:TUNDRA]
[BIOME:FOREST_TAIGA]
[BIOME:ANY_TEMPERATE_FOREST]
[BIOME:SHRUBLAND_TEMPERATE]
[BIOME:MOUNTAIN]
[POPULATION_NUMBER:15:30]
[CLUSTER_NUMBER:3:7]
[GRASSTRAMPLE:0][NATURAL]
[PETVALUE:1000]
[PET_EXOTIC]
[TRAINABLE]
[GOOD]
[FLIER]
[GRASSTRAMPLE:0]
[MOUNT_EXOTIC]
[FANCIFUL]
[BONECARN]
[PREFSTRING:elaborate courtship flights]
[PREFSTRING:noble features]
[PREFSTRING:devoted parenting]
[BODY:QUADRUPED_NECK:2WINGS:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:4TOES_FQ_REG:4TOES_RQ_REG:MOUTH:TONGUE:GENERIC_TEETH_WITH_LARGE_EYE_TEETH:RIBCAGE]
[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:FEATHER:FEATHER_TEMPLATE]
[USE_TISSUE_TEMPLATE:FEATHER:FEATHER_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:WING]
[USE_MATERIAL_TEMPLATE:TALON:NAIL_TEMPLATE]
[USE_TISSUE_TEMPLATE:TALON:TALON_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:TOE:TALON: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:0:0:20000]
[BODY_SIZE:1:0:100000]
[BODY_SIZE:2:0:200000]
[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:20:40]
[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_PREPARE_AND_RECOVER:3:3]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[ATTACK:SCRATCH:CHILD_TISSUE_LAYER_GROUP:BY_TYPE:STANCE:BY_CATEGORY:ALL:TALON]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:claw:claws]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PREPARE_AND_RECOVER:3:3]
[ATTACK_PRIORITY:SECOND]
[DIURNAL]
[HOMEOTHERM:10070]
[APPLY_CREATURE_VARIATION:STANDARD_QUADRUPED_GAITS:900:447:298:149:1900:2900] 59 kph
[APPLY_CREATURE_VARIATION:STANDARD_SWIMMING_GAITS:6561:6115:5683:1755:7456:8567] 5 kph
[APPLY_CREATURE_VARIATION:STANDARD_CRAWLING_GAITS:6561:6115:5683:1755:7456:8567] 5 kph
[SWIMS_INNATE]
[MUNDANE]
[CASTE:FEMALE]
[FEMALE]
[CASTE:MALE]
[MALE]
[SET_BP_GROUP:BY_TYPE:LOWERBODY][BP_ADD_TYPE:GELDABLE]
[SELECT_CASTE:ALL]
[SET_TL_GROUP:BY_CATEGORY:ALL:HAIR]
[TL_COLOR_MODIFIER:WHITE:1:SILVER:1:PEARL:1]
[TLCM_NOUN:hair and feathers:PLURAL]
[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_AMBER:1:ORANGE:1:GOLD:1]
[TLCM_NOUN:eyes:PLURAL]
[SELECT_MATERIAL:ALL]
[MULTIPLY_VALUE:4]
Of course, considering that a wolf with bird feet is the polar opposite of a Goomy, this won't 100% apply.
EDIT: The pooka ruins the game too, confirming that the mere act of adding a creature, any creature, that I didn't add in 40.19 ruins it, but there is nothing wrong with the creatures themselves, or even their positioning. Not even putting them in their own file helped. Could it have something to do with the fact that I had to delete one of the libs for dfhack to work? The fact that I'm on OSX and Mac users aren't allowed to have nice things? I'll have to do a third install of DF, put in Phoebus without DFHack, try the bison there, move it to my "real" DF folder, and see what happens.
EDIT: Even when put in a 100% fresh and new install of DF with literally nothing - NOTHING! - altered about it, it still caused the problem in both cases. There were no graphics, no DFHack, no deleted libs, literally nothing else. The errorlogs have remained gleefully empty the entire time. I don't think it's possible to find out what the problem is, let alone to fix it. I've got a sinking feeling that it's because I'm on OSX, and that's the one variable I cannot change.
I think this is it for me, fellow modders. There's no use creating mods if you can't even use them without ruining your game. I am bitterly disappointed.