Spread life and death.
Henzell says...
  1. From worst to best: Brainless, Animal, Human. This has many effects on monster behaviour, most notably pursuing you longer, pathfinding better, being more wary of traps/clouds and moving in ways that inconviences you more.
  2. Stealth: As intelligence increases: *Forgets about you slower if you leave the level. *Remembers you longer if you leave LoS *Passes in-LoS stealth checks better. Also, Human are better at guessing the location of an invisible player, and Animal- relaxes their guard faster when wandering with you as their target.
  3. Helping out friends: Human/Animal-intelligence monsters move out of the way if further along the same 8-way line from the player there is a same-genus monster with a ranged attack, or a same-genus monster immediately behind them with a melee attack.
  4. The following entries are out-of-date; please correct them if you have the energy.
  5. Pathfinding: Smarter monsters have longer pathfinding range - 2 for plant, 4 for insect/reptile, 5 for animal, 8 for normal, infinite for high. (+= 4 for cling, += 3 for native to branch, +1 for blood scent, *5 for ash penance, *5 for mark.) Animal- doesn't pathfind around shallow water it would flounder in. Animal+ if it can't pathfind to you and
  6. Ranged AI: Normal+ that has ranged attack (and that isn't 'fighter' tagged except for centaur warriors and yaktaur captains), when you leave their LoS, will 9/10 (19/20 for High) move to the position that maximizes their distance from you instead of directly towards you.
  7. Traps: High or (Insect+ and native to branch) or (allied and you know location) cannot fall through a shaft. Native to branch or (high and 1/3 chance) knows the existence of a trap. Mindless creatures are not afraid of traps.
  8. Hazards: Animal+ avoids clouds they are extra-weak to (+tornado/calcifying dust). Plant is not afraid of clouds. High won't move while confused if they might drown/melt in lava. Reptile- is not afraid of slime walls.
  9. Misc stuff: Animal- can be distracted by noise (ME_DISTURB) even when alert. Animal- berserk enemies just attack whatever's closest instead of preferring you. Normal+ monsters guess your MR and abort casting a hex if it seems pointless (High estimates better). Normal+ monsters dip themselves into water to extinguish sticky flame.
  10. Misc stuff: Animal- can't be recalled by other monsters. Animal- except spectral weapons is considered to have 0 skills. Plant can't frenzy or jump. Animal- is immune to intoxication. Insect- is immune to starcursed mass screams. Dumb monsters are worse at 'patrolling'.
  11. More...

Information based on master branch and the great LearnDB.
© Dungeon Crawl Stone Soup, 2010.