So I used the snapping turtle raw as a base. These subterranean tortoises are giant, have rock for shells, and have magma for blood (Surprisingly harmless). They can also spit magma at enemies (Much less harmless).
They have the exotic pet tag, in case someone wanted to make turrets out of them or something.
[CREATURE:FURNACE TORTOISE]
[DESCRIPTION:A giant reptile with a thick rock shell, which it can retreat into when threatened. As large as a bear, these creatures sustain a molten core in their bodies. They can regurgitate some magma as a defense.]
[NAME:furnace tortoise:furnace tortoises:furnace tortoise]
[CASTE_NAME:furnace tortoise:furnace tortoises:furnace tortoise]
[CREATURE_TILE:'T'][COLOR:4:0:0]
[CARNIVORE]
[PET_EXOTIC]
[PETVALUE:2500]
[NOT_BUTCHERABLE]
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Spit Magma]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:BEAK]
[CDI:MATERIAL:INORGANIC:OBSIDIAN:LIQUID_GLOB]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:50]
[BIOME:SUBTERRANEAN_CHASM]
[POPULATION_NUMBER:0:20]
[PREFSTRING:likeness to a volcano]
[BODY:QUADRUPED_NECK:TAIL:2EYES:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:SKULL:BEAK:RIBCAGE:SHELL]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[REMOVE_MATERIAL:SKIN]
[REMOVE_MATERIAL:LEATHER]
[REMOVE_MATERIAL:HAIR]
[USE_MATERIAL_TEMPLATE:SCALE:SCALE_TEMPLATE]
[USE_MATERIAL_TEMPLATE:SHELL:SHELL_TEMPLATE]
[STATE_COLOR:ALL:DARK_GREEN]
[TISSUE:OBSIDIAN]
[TISSUE_NAME:obsidian:obsidian]
[TISSUE_MATERIAL:INORGANIC:OBSIDIAN]
[STRUCTURAL]
[RELATIVE_THICKNESS:2]
[HEALING_RATE:1000]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[SETTABLE]
[SPLINTABLE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[REMOVE_TISSUE:SKIN]
[REMOVE_TISSUE:HAIR]
[USE_TISSUE_TEMPLATE:SCALE:SCALE_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:SHELL:OBSIDIAN]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SCALE:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:LEATHERY_EGG_MATERIALS]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SCALE:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:SHELL_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]
[SELECT_MATERIAL:ALL]
[HEATDAM_POINT:NONE]
[IGNITE_POINT:NONE]
[MELTING_POINT:NONE]
[APPLY_CREATURE_VARIATION:STANDARD_WALK_CRAWL_GAITS:6561:6115:5683:1755:7456:8567] 5 kph
[APPLY_CREATURE_VARIATION:STANDARD_CRAWLING_GAITS:9000:8900:8825:8775:9500:9900] 1 kph
[APPLY_CREATURE_VARIATION:STANDARD_SWIMMING_GAITS:900:734:568:366:1900:2900] 24 kph
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:INORGANIC:OBSIDIAN: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:0:0:2000]
[BODY_SIZE:4:0:200000]
[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:30:50]
[ATTACK:BITE:BODYPART:BY_CATEGORY:BEAK]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_EDGE]
[ATTACK_PREPARE_AND_RECOVER:3:3]
[ATTACK_FLAG_CANLATCH]
[DIURNAL]
[CANNOT_JUMP]
[CASTE:FEMALE]
[FEMALE]
[LAYS_EGGS]
[EGG_MATERIAL:LOCAL_CREATURE_MAT:EGGSHELL:SOLID]
[EGG_MATERIAL:LOCAL_CREATURE_MAT:EGG_WHITE:LIQUID]
[EGG_MATERIAL:LOCAL_CREATURE_MAT:EGG_YOLK:LIQUID]
[EGG_SIZE:2000]
[CLUTCH_SIZE:2:4] actually much higher
[CASTE:MALE]
[MALE]
[SELECT_CASTE:ALL]
[SET_TL_GROUP:BY_CATEGORY:ALL:SCALE]
[TL_COLOR_MODIFIER:DARK_GREEN:1]
[TLCM_NOUN:scales:PLURAL]
[SET_TL_GROUP:BY_CATEGORY:EYE:EYE]
[TL_COLOR_MODIFIER:BLACK:1]
[TLCM_NOUN:eyes:PLURAL]