I made some raws and body parts for the elusive Chimaera. It might need some playtesting in actual worldgen, since I've only used it in arena mode so far, but the errorlog is clear, at least! It is a megabeast, it's fire-breathing and venomous, but lacks the goat head (mainly because it's useless, but I could easily add it). Here:
creature raws[OBJECT:CREATURE]
[CREATURE:MOD_ALFI_CHIMAERA]
[DESCRIPTION:A divine being, for it has the head of a lion and the tail of a serpent, while its body is that of a goat, and it breathes forth flames of fire.]
[NAME:chimaera:chimaera:chimaera]
[CASTE_NAME:chimaera:chimaera:chimaera]
[CREATURE_TILE:'C'][COLOR:4:0:1]
[PETVALUE:5000]
[PET_EXOTIC]
[TRAINABLE]
[MOUNT_EXOTIC]
[BIOME:ANY_LAND]
[FREQUENCY:5]
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Breathe fire]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:MOUTH]
[CDI:FLOW:FIREJET]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:50]
[FIREIMMUNE]
[FANCIFUL]
[LARGE_PREDATOR]
[MEGABEAST][DIFFICULTY:10]
[ATTACK_TRIGGER:80:10000:100000]
[SPHERE:FIRE]
[SPHERE:BLIGHT]
[SPHERE:CHAOS]
[NOFEAR][NOEXERT][NOPAIN][NOSTUN]
[BUILDINGDESTROYER:2]
[GRASSTRAMPLE:50]
[BONECARN]
[PREFSTRING:multifarious lethality]
[LARGE_ROAMING]
[BODY:QUADRUPED_NECK_HOOF_HEADLESS:CHIMAERA_LION_HEAD:TAIL:CHIMAERA_SERPENT_HEAD:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:MOUTH:GENERIC_TEETH:TONGUE:RIBCAGE]
[NATURAL]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[USE_MATERIAL_TEMPLATE:HOOF:HOOF_TEMPLATE]
[USE_MATERIAL_TEMPLATE:SCALE:SCALE_TEMPLATE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[USE_TISSUE_TEMPLATE:HOOF:HOOF_TEMPLATE]
[USE_TISSUE_TEMPLATE:SCALE:SCALE_TEMPLATE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:BODY_HAIR_TISSUE_LAYERS:HAIR]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SKIN:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[SELECT_TISSUE_LAYER:SKIN:BY_CATEGORY:TAIL]
[PLUS_TISSUE_LAYER:SKIN:BY_TOKEN:CSHD]
[SET_LAYER_TISSUE:SCALE]
[RELSIZE:BY_CATEGORY:TAIL:200]
[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]
[USE_MATERIAL_TEMPLATE:VENOM:CREATURE_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen chimaera venom]
[STATE_ADJ:ALL_SOLID:frozen chimaera venom]
[STATE_NAME:LIQUID:chimaera venom]
[STATE_ADJ:LIQUID:chimaera venom]
[STATE_NAME:GAS:boiling chimaera venom]
[STATE_ADJ:GAS:boiling chimaera venom]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:chimaera bite]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:CHIMAERA:ALL]
[SYN_INJECTED]
[CE_NECROSIS:SEV:100:PROB:100:RESISTABLE:BP:BY_TYPE:THOUGHT:ALL:BP:BY_TYPE:NERVOUS:ALL:START:30:PEAK:60:END:1200]
[BODY_SIZE:0:0:50000]
[BODY_SIZE:100:0:15000000]
[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:5000:7500]
[DIURNAL]
[HOMEOTHERM:10070]
[APPLY_CREATURE_VARIATION:STANDARD_QUADRUPED_GAITS:900:438:292:146:1900:2900] 60 kph
[APPLY_CREATURE_VARIATION:STANDARD_SWIMMING_GAITS:2990:2257:1525:731:4300:6100] 12 kph
[APPLY_CREATURE_VARIATION:STANDARD_CRAWLING_GAITS:2990:2257:1525:731:4300:6100] 12 kph
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_TOKEN:CLHD: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_WITH]
[ATTACK_FLAG_CANLATCH]
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_TOKEN:CSHD: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_WITH]
[ATTACK_FLAG_CANLATCH]
[SPECIALATTACK_INJECT_EXTRACT:LOCAL_CREATURE_MAT:VENOM:LIQUID:100:100]
[ATTACK:KICK:BODYPART:BY_CATEGORY:HOOF_FRONT]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:4:4]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_WITH]
[ATTACK_FLAG_BAD_MULTIATTACK]
[ATTACK:KICK:BODYPART:BY_CATEGORY:HOOF_REAR]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:4:4]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_WITH]
[ATTACK_FLAG_BAD_MULTIATTACK]
[SWIMS_INNATE]
[LAIR:WILDERNESS_LOCATION:100]
[LAIR_HUNTER]
[MULTIPART_FULL_VISION]
[NATURAL_SKILL:BITE:6]
[NATURAL_SKILL:STANCE_STRIKE:6]
[NATURAL_SKILL:RANGED_COMBAT:6]
[NATURAL_SKILL:MELEE_COMBAT:6]
[NATURAL_SKILL:DODGING:6]
[NATURAL_SKILL:SITUATIONAL_AWARENESS:6]
[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:GOLD:1:BLACK:1:YELLOW:1:BROWN:1:COPPER:1]
[TLCM_NOUN:hair:SINGULAR]
[SET_TL_GROUP:BY_CATEGORY:ALL:SKIN]
[TL_COLOR_MODIFIER:BROWN:1:CINNAMON:1:COPPER:1:DARK_BROWN:1:PALE_BROWN:1:PALE_CHESTNUT:1:PALE_PINK:1:PINK:1:RAW_UMBER:1:TAN:1]
[TLCM_NOUN:skin:SINGULAR]
[SET_TL_GROUP:BY_CATEGORY:ALL:SCALE]
[TL_COLOR_MODIFIER:GREEN:1:TAN:1:BLACK:1]
[TLCM_NOUN:scales:PLURAL]
[SET_TL_GROUP:BY_CATEGORY:EYE:EYE]
[TL_COLOR_MODIFIER:RED:1:GOLD:1]
[TLCM_NOUN:eyes:PLURAL]
[SELECT_MATERIAL:ALL]
[MULTIPLY_VALUE:12]
[COLDDAM_POINT:NONE]
[HEATDAM_POINT:NONE]
[IGNITE_POINT:NONE]
[IF_EXISTS_SET_MELTING_POINT:15000]
[IF_EXISTS_SET_BOILING_POINT:20000]
[SELECT_MATERIAL:BLOOD]
[PLUS_MATERIAL:PUS]
[MELTING_POINT:10000]
body parts[OBJECT:BODY]
[BODY:CHIMAERA_LION_HEAD]
[BP:CLHD:lion head:STP][CON:NK][HEAD][CATEGORY:HEAD]
[DEFAULT_RELSIZE:400]
[BP:R_EYE_TOOTH:right eye tooth:right eye teeth][CON:CLHD][CATEGORY:TOOTH][SOCKET][SMALL]
[DEFAULT_RELSIZE:15]
[BP:L_EYE_TOOTH:left eye tooth:left eye teeth][CON:CLHD][CATEGORY:TOOTH][SOCKET][SMALL]
[DEFAULT_RELSIZE:15]
[BP:R_EAR:right ear:STP][CON:CLHD][HEAR][SMALL][RIGHT][CATEGORY:EAR]
[DEFAULT_RELSIZE:15]
[BP:L_EAR:left ear:STP][CON:CLHD][HEAR][SMALL][LEFT][CATEGORY:EAR]
[DEFAULT_RELSIZE:15]
[BODY:CHIMAERA_SERPENT_HEAD]
[BP:CSHD:serpent head:STP][CON:TAIL][HEAD][CATEGORY:HEAD]
[DEFAULT_RELSIZE:200]
[BP:R_FANG:right fang:right fangs][CON:CSHD][CATEGORY:TOOTH][SOCKET][SMALL]
[DEFAULT_RELSIZE:10]
[BP:L_FANG:left fang:left fangs][CON:CSHD][CATEGORY:TOOTH][SOCKET][SMALL]
[DEFAULT_RELSIZE:10]
[BODY:QUADRUPED_NECK_HOOF_HEADLESS]
[BP:UB:upper body:upper bodies][UPPERBODY][CATEGORY:BODY_UPPER]
[DEFAULT_RELSIZE:1000]
[BP:LB:lower body:lower bodies][CON:UB][LOWERBODY][CATEGORY:BODY_LOWER]
[DEFAULT_RELSIZE:1000]
[BP:NK:neck:STP][CON:UB][CATEGORY:NECK]
[DEFAULT_RELSIZE:150]
[BP:RA:right front leg:STP][CON:UB][LIMB][RIGHT][CATEGORY:LEG_FRONT]
[DEFAULT_RELSIZE:900]
[BP:LA:left front leg:STP][CON:UB][LIMB][LEFT][CATEGORY:LEG_FRONT]
[DEFAULT_RELSIZE:900]
[BP:RH:right front hoof:right front hooves][CON:RA][STANCE][RIGHT][CATEGORY:HOOF_FRONT]
[DEFAULT_RELSIZE:120]
[BP:LH:left front hoof:left front hooves][CON:LA][STANCE][LEFT][CATEGORY:HOOF_FRONT]
[DEFAULT_RELSIZE:120]
[BP:RL:right rear leg:STP][CON:LB][LIMB][RIGHT][CATEGORY:LEG_REAR]
[DEFAULT_RELSIZE:900]
[BP:LL:left rear leg:STP][CON:LB][LIMB][LEFT][CATEGORY:LEG_REAR]
[DEFAULT_RELSIZE:900]
[BP:RF:right rear hoof:right rear hooves][CON:RL][STANCE][RIGHT][CATEGORY:HOOF_REAR]
[DEFAULT_RELSIZE:120]
[BP:LF:left rear hoof:left rear hooves][CON:LL][STANCE][LEFT][CATEGORY:HOOF_REAR]
[DEFAULT_RELSIZE:120]
Report any bugs/tweaks, I learned a lot creating this one!