Would you mind copy/pasting the raws here, so we can see the issue? Much easier to diagnose problems that way
Completely slipped my mind.
[CREATURE:ARACHNID]
[DESCRIPTION:An evil spider thing.]
[NAME:arachnid:arachnids:arachnia]
[CASTE_NAME:arachnid:arachnid:arachnia]
[TILE:'&'][COLOR:7:0:0]
[MODVALUE:20][SEMIMEGABEAST]
[INTELLIGENT][CANOPENDOORS]
[LARGE_CHASM][FREQUENCY:1][DIFFICULTY:4]
[SPEED:700]
[STOUT]
[EXTRACT:arachnid venom:7:0:0]
[EXTRACT_VALUE:500]
[EXTRACT_PARALYZE][NATURAL][PET_EXOTIC]
[EXTRACT_ANTIDOTE:arachnid antivenin:7:0:0:100]
[WEBIMMUNE][AMBUSHPREDATOR][PARALYZEIMMUNE]
[GENPOWER:4]
[BLOODTYPE:W][CHITIN]
[PETVALUE:2500]
[GRASSTRAMPLE:10]
[CARNIVORE][BONECARN]
[EXTRAVISION][NOSTUN][NOEXERT][NOFEAR]
[BUILDINGDESTROYER:1]
[PREFSTRING:terrifying presence]
[PREFSTRING:fearsome venom]
[BODY:ARACHNOBODY:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:THROAT:NECK:SPINE:BRAIN:MOUTH:5FINGERS]
[SIZE:15]
[MAXAGE:20:30]
[ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:1:6:GORE][SPECIALATTACK_INJECT_EXTRACT:50:100]
[ATTACK:SECOND:BYTYPE:GRASP:punch:punches:1:2:BLUDGEON][ATTACKFLAG_WITH]
[FAT:2]
[CHILD:30][BABY:1][MULTIPLE_LITTER_RARE]
[DAMBLOCK:10]
[ALL_ACTIVE][CAVE_ADAPT]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[LAYERING:50]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[MUNDANE]
(not sure if entity_default entries are relevant here but, included anyway)
[ENTITY:MOUNTAIN]
[MOUNTAIN_SETTLEMENTS]
[CREATURE:ARACHNID]
[INDIV_CONTROLLABLE]
[ADVENTURE_TIER:5]
[TRANSLATION:ELF]
[BIOME_SUPPORT:ANY_WETLAND:2]
[BIOME_SUPPORT:ANY_DESERT:3]
[BIOME_SUPPORT:ANY_FOREST:1]
[BIOME_SUPPORT:MOUNTAIN:4]
[BIOME_SUPPORT:ANY_GRASSLAND:1]
[BIOME_SUPPORT:ANY_SAVANNA:1]
[BIOME_SUPPORT:ANY_SHRUBLAND:1]
[BIOME_SUPPORT:ANY_RIVER:2]
[START_BIOME:MOUNTAIN]
[SELECT_SYMBOL:ALL:EVIL]
[SELECT_SYMBOL:ALL:VIOLENT]
[SELECT_SYMBOL:ALL:DEATH]
[SELECT_SYMBOL:ALL:UNTOWARD]
[CULL_SYMBOL:ALL:DOMESTIC]
[CULL_SYMBOL:ALL:FLOWERY]
[CULL_SYMBOL:ALL:HOLY]
[CULL_SYMBOL:ALL:PEACE]
[CULL_SYMBOL:ALL:NEGATOR]
[CULL_SYMBOL:ALL:GOOD]
[FRIENDLY_COLOR:7:0:0]
[DEFAULT_SITE_TYPE:CAVE]
[LIKES_SITE:CAVE]
[TOLERATES_SITE:CAVE]
[ACTIVE_SEASON:SPRING]
[ACTIVE_SEASON:SUMMER]
[ACTIVE_SEASON:AUTUMN]
[ACTIVE_SEASON:WINTER]
[MAX_STARTING_CIV_NUMBER:2]
[START_GROUP_NUMBER:1]
[MAX_POP_NUMBER:500]
[MAX_SITE_POP_NUMBER:20]
[ETHIC:KILL_ENTITY_MEMBER:ACCEPTABLE]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:ACCEPTABLE]
[ETHIC:TORTURE_FOR_INFORMATION:ACCEPTABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:ACCEPTABLE]
[ETHIC:TREASON:ACCEPTABLE]
[ETHIC:OATH_BREAKING:ACCEPTABLE]
[ETHIC:LYING:ACCEPTABLE]
[ETHIC:VANDALISM:ACCEPTABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:ACCEPTABLE]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:ACCEPTABLE]
[ETHIC:EAT_SAPIENT_KILL:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:ACCEPTABLE]
I've already noted the creature is missing various things. It lacks tissues (rendering it unable to actually do much of anything) and description modifiers (indeed, attempting to view its description crashes the game) but I don't see how either of those would create this problem.
Though Putnam helpfully pointed out the guide I'd used is miserably outdated so the easiest fix is obviously just to edit an existing creature using the current guide instead. So I suppose that's that.
I'd still like to figure out what this exact flaw is in case I ever decide to build a creature from scratch rather than with editing.