I just added in my first custom creature. It's called a fluffball (babies are fuzzballs), and it is nigh-invulnerable to everything I throw at it. The actual idea I borrowed [read:shamelessly ripped off] from someone else's "featherball," but I added a great deal of the code myself, mainly to make it usable in fortress mode.
[CREATURE:FLUFFBALL]
[DESCRIPTION:An adorable ball of fluff.]
[NAME:fluffball:fluffballs:fluffball]
[CASTE_NAME:fluffball:fluffballs:fluffball]
[CREATURE_TILE:'o'][COLOR:7:0:1]
[BIOME:ANY_LAND]
[BENIGN]
[LARGE_ROAMING]
[DIURNAL]
[MAXAGE:5:7]
[MUNDANE]
[NATURAL_SKILL:PROSE:10]
[PET][GOOD]
[CANNOT_UNDEAD]
[COMMON_DOMESTIC]
[PETVALUE:30]
[TRAINABLE]
[LITTERSIZE:1:2]
[CHILD:3]
[GENERAL_CHILD_NAME:fuzzball:fuzzballs]
[FREQUENCY:10][POPULATION_NUMBER:20:60][CLUSTER_NUMBER:5:10]
[EXTRAVISION]
[FLIER]
[WEBIMMUNE]
[NOPAIN][NOEXERT][NO_FEVERS][NOBREATHE][NOSTUN]
[NO_DRINK][NO_EAT][ALL_ACTIVE]
[NO_THOUGHT_CENTER_FOR_MOVEMENT][NOBONES]
[NOT_BUTCHERABLE]
[PREFSTRING:fluffyness]
[PREFSTRING:adorable cuteness]
[PREFSTRING:sleeping dust]
[BODY_SIZE:0:0:100]
[BODY_SIZE:0:100:250]
[BODY_SIZE:1:10:500]
[BODY:BASIC_1PARTBODY]
[SPEED:1750]
[USE_MATERIAL_TEMPLATE:FLUFF:FEATHER_TEMPLATE]
[MAT_FIXED_TEMP:10067]
[STATE_NAME_ADJ:ALL:fluff]
[STATE_COLOR:ALL:WHITE]
[DISPLAY_COLOR:7:0:1]
[SOLID_DENSITY:1]
[USE_MATERIAL_TEMPLATE:POWDER:CREATURE_EXTRACT_TEMPLATE]
[MAT_FIXED_TEMP:10067]
[STATE_NAME_ADJ:ALL:powder]
[STATE_COLOR:ALL:WHITE]
[DISPLAY_COLOR:7:0:1]
[SOLID_DENSITY:1]
[MELTING_POINT:10000]
[BOILING_POINT:10002]
[SYNDROME]
[SYN_NAME:fluffy comfyness]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:FLUFFBALL]
[SYN_CONTACT]
[SYN_INHALED]
[CE_DROWSINESS:SEV:300:PROB:90:START:5:PEAK:10:END:1000]
[CE_DIZZINESS:SEV:300:PROB:60:START:5:PEAK:30:END:1500]
[CE_NUMBNESS:SEV:300:PROB:30:START:5:PEAK:200:END:1500]
[CE_PARALYSIS:SEV:100:PROB:1:START:5:PEAK:300:END:5000]
[TISSUE:FLUFF]
[TISSUE_NAME:fluff:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:FLUFF]
[RELATIVE_THICKNESS:1]
[TISSUE_LEAKS]
[TISSUE_SHAPE:FEATHERS]
[VASCULAR:1]
[TISSUE_LAYER:BY_CATEGORY:ALL:FLUFF]
[ATTACK:FLUFF:BODYPART:BY_TOKEN:UB]
[ATTACK_SKILL:PROSE]
[ATTACK_VERB:fluff:fluffs]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PRIORITY:MAIN]
[SPECIALATTACK_INJECT_EXTRACT:LOCAL_CREATURE_MAT:POWDER:POWDER:100:100]
[ATTACK_VELOCITY_MODIFIER:1]
[SECRETION:LOCAL_CREATURE_MAT:POWDER:POWDER:BY_CATEGORY:ALL:ALL]
[MATERIAL_BREATH_ATTACK:LOCAL_CREATURE_MAT:POWDER:TRAILING_GAS_FLOW]
[SOUND:ALERT:200:1000:VOCALIZATION:squeak:squeaks:an alarmed squeak]
[SOUND:PEACEFUL_INTERMITTENT:10:1000:VOCALIZATION:purr:purrs:deep purring]
[CASTE:FEMALE]
[FEMALE]
[CASTE:MALE]
[MALE]
[SELECT_CASTE:FEMALE]
[USE_MATERIAL_TEMPLATE:MILK:MILK_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen fluff milk]
[STATE_ADJ:ALL_SOLID:frozen fluff milk]
[STATE_NAME:LIQUID:fluff milk]
[STATE_ADJ:LIQUID:fluff milk]
[STATE_NAME:GAS:boiling fluff milk]
[STATE_ADJ:GAS:boiling fluff milk]
[PREFIX:NONE]
[MILKABLE:LOCAL_CREATURE_MAT:MILK:200]
[USE_MATERIAL_TEMPLATE:CHEESE:CREATURE_CHEESE_TEMPLATE]
[STATE_NAME:SOLID:fluff cheese]
[STATE_ADJ:SOLID:fluff cheese]
[STATE_NAME:SOLID_POWDER:fluff cheese powder]
[STATE_ADJ:SOLID_POWDER:fluff cheese powder]
[STATE_NAME:LIQUID:melted fluff cheese]
[STATE_ADJ:LIQUID:melted fluff cheese]
[STATE_NAME:GAS:boiling fluff cheese]
[STATE_ADJ:GAS:boiling fluff cheese]
[PREFIX:NONE]
Let's see, there's a syndrome that quite effectively incapacitates almost anything, in a powder that they shed whenever it rains. They're milkable, they breed incredibly fast, they purr/squeak, and setting them on fire
only makes them deadlier. Did I mention that they're tiny and fluffy and RELENTLESS?
EDIT: The latest version of the code! I made it so they can't be undead, because that caused some bizarre, game-breaking errors.