Something like this would be better posed in the modding board.
Anyway, you could try adding a megabeast entity, then starting an adventure as one of them. I'll work on a little guide and post it here sometime.
[EDIT]
Here is my powerful creature of death and doom and destruction:
[CREATURE:MEGATAUR]
[DESCRIPTION:A giant humanoid monster with the head of a bull. It is absurdly powerful.]
[NAME:megataur:megataurs:megataur]
[CASTE_NAME:megataur:megataurs:megataur]
[CREATURE_TILE:'M'][COLOR:5:0:0]
[FANCIFUL]
[MEGABEAST][DIFFICULTY:15]
[ATTACK_TRIGGER:50:5000:50000]
[CAN_LEARN][CAN_SPEAK]
[CANOPENDOORS]
[BUILDINGDESTROYER:2]
[PREFSTRING:horns]
[BIOME:ANY_LAND]
[SPHERE:CAVERNS]
[SPHERE:CHAOS]
[SPHERE:DARKNESS]
[SPHERE:DEFORMITY]
[SPHERE:STRENGTH]
[FLIER]
[BODY:HUMANOID:2WINGS:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:SKULL:2HEAD_HORN:5FINGERS:5TOES:MOUTH:TONGUE:EYELIDS:CHEEKS:TEETH:RIBCAGE]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[USE_MATERIAL_TEMPLATE:HORN:HORN_TEMPLATE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[USE_TISSUE_TEMPLATE:HORN:HORN_TEMPLATE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[TISSUE_LAYER:BY_CATEGORY:HEAD:HAIR:NORMAL]
[USE_TISSUE_TEMPLATE:EYEBROW:EYEBROW_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:HEAD:EYEBROW:ABOVE:BY_CATEGORY:EYE]
[USE_TISSUE_TEMPLATE:EYELASH:EYELASH_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:EYELID:EYELASH:FRONT]
[USE_MATERIAL_TEMPLATE:NAIL:NAIL_TEMPLATE]
[USE_TISSUE_TEMPLATE:NAIL:NAIL_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:FINGER:NAIL:FRONT]
[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_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RELSIZES]
[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:0:0:10000]
[BODY_SIZE:1:168:50000]
[BODY_SIZE:12:0:220000]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:GORE:BODYPART:BY_CATEGORY:HORN]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:gore:gores]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ATTACK:SCRATCH:CHILD_TISSUE_LAYER_GROUP:BY_TYPE:GRASP:BY_CATEGORY:FINGER:NAIL]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
[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:SECOND]
[ATTACK_FLAG_CANLATCH]
[BABY:1]
[CHILD:12]
[EQUIPS]
[NATURAL_SKILL:WRESTLING:12]
[NATURAL_SKILL:BITE:12]
[NATURAL_SKILL:GRASP_STRIKE:12]
[NATURAL_SKILL:STANCE_STRIKE:12]
[NATURAL_SKILL:MELEE_COMBAT:12]
[NATURAL_SKILL:DODGING:12]
[NATURAL_SKILL:AXE:2]
[NATURAL_SKILL:SWORD:12]
[NATURAL_SKILL:DAGGER:12]
[NATURAL_SKILL:PIKE:12]
[NATURAL_SKILL:MACE:12]
[NATURAL_SKILL:HAMMER:12]
[NATURAL_SKILL:WHIP:12]
[NATURAL_SKILL:SPEAR:12]
[NATURAL_SKILL:SITUATIONAL_AWARENESS:12]
[PHYS_ATT_RANGE:STRENGTH:2000:2100:2150:2250:2350:2550:3250]
[PHYS_ATT_RANGE:TOUGHNESS:2000:2100:2150:2250:2350:2550:3250]
[PHYS_ATT_RANGE:AGILITY:2000:2100:2150:2250:2350:2550:3250]
[PHYS_ATT_RANGE:ENDURANCE:2000:2100:2150:2250:2350:2550:3250]
[PHYS_ATT_RANGE:RECUPERATION:2000:2100:2150:2250:2350:2550:3250]
[PHYS_ATT_RANGE:DISEASE_RESISTANCE:2000:2100:2150:2250:2350:2550:3250]
[MENT_ATT_RANGE:FOCUS:2000:2100:2150:2250:2350:2550:3250]
[MENT_ATT_RANGE:WILLPOWER:2000:2100:2150:2250:2350:2550:3250]
[MENT_ATT_RANGE:MEMORY:3000:3100:3150:3250:3350:3550:4250]
[MENT_ATT_RANGE:SPATIAL_SENSE:3000:3100:3150:3250:3350:3550:4250]
[MENT_ATT_RANGE:KINESTHETIC_SENSE:2000:2100:2150:2250:2350:2550:3250]
[LAIR:LABYRINTH:100]
[LAIR_HUNTER]
[LAIR_HUNTER_SPEECH:lair_hunter_minotaur.txt]
[HABIT_NUM:TEST_ALL]
[HABIT:GRIND_BONE_MEAL:100]
[HABIT:EAT_BONE_PORRIDGE:100]
[HABIT:USE_ANY_MELEE_WEAPON:100]
[ALL_ACTIVE]
[HOMEOTHERM:10067]
[SWIMS_INNATE][SWIM_SPEED:2500]
[CASTE:FEMALE]
[FEMALE]
[MULTIPLE_LITTER_RARE]
[CASTE:MALE]
[MALE]
[SELECT_CASTE:ALL]
[SET_TL_GROUP:BY_CATEGORY:ALL:HAIR]
[TL_COLOR_MODIFIER:BROWN: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:BLACK:1]
[TLCM_NOUN:eyes:PLURAL]
Start by adding the above code to creature_standard.txt. This will mean that megataurs will appear in the next world you create, so you can skip this step is you don't want my epic creature.
Next, add this to entity_standard.txt:
[ENTITY:MONSTER]
[INDIV_CONTROLLABLE]
[CREATURE:MEGATAUR]
You can add other CREATURE: tags to this entity, like [CREATURE:DRAGON] or [CREATURE:SPIDER_CAVE_GIANT] and you should be able to adventure as them in your next world. If you skipped the first step, don't add the [CREATURE:MEGATAUR] tag or nonsense will ensue.