I released the alpha with this property included. I forgot to include the updated imp.cfg which commented the new property, but it functions all the same.

; Creature properties
;   BLEEDS - the creature leaves blood when is hit, slapped, dying or being injured in any way
;   UNAFFECTED_BY_WIND - the creature isn't pushed back by Wind spell
;   IMMUNE_TO_GAS - the creature isn't injured by Gas Traps and Farts
;   HUMANOID_SKELETON - the creature leaves a skeleton if left in prison to die
;   PISS_ON_DEAD - creature feels urge to piss on nearby dead bodies
;   FLYING - creature normally isn't touching ground when moving and can move up and down
;   SEE_INVISIBLE - creature has natural ability which works like Sight spell
;   PASS_LOCKED_DOORS - creature can move through locked doors and won't fight with doors
;   SPECIAL_DIGGER - creature can dig and perform other dungeon tasks
;   ARACHNID - creature is kind of spider
;   DIPTERA - creature is kind of fly
;   LORD - creature is lord of the land, usually arrives to level as final boss, and at arrival
;       you can hear "beware, the lord of the land approaches"
;   SPECTATOR - creature is just a spectator for multiplayer games
;   EVIL - creature has evil nature
;   NEVER_CHICKENS - creature isn't affected by Chicken spell
;   IMMUNE_TO_BOULDER - when boulder trap hits the creature, falls apart without dealing any damage
;   NO_CORPSE_ROTTING - Creature body can't be taken to rot on graveyard
;   NO_ENMHEART_ATTCK - Creature won't attack enemy dungeon heart on sight
;   TREMBLING_FAT - Creature is so fat that ground trembles when it falls
;   FEMALE - Creature is a female, does female sounds and has female name
;   INSECT - Creature is an insect (note that DIPTERA and ARACHNID creatures should also have INSECT set explicitly)
;   ONE_OF_KIND - Only one creature of that kind may exist on a specific map. Creature name is set to kind name.
;   NO_IMPRISONMENT - Creature cannot be stunned for prison.
;   IMMUNE_TO_DISEASE - Creature cannot get diseased