This data was retrieved from the default ini files, running Terraria v1.1.2.

List of NPC AI Styles

ID Name Description
0 Null Used by bound NPCs.
1 Slime Used by various slimes (except King Slime).
2 Simple Flier Used by Demon Eye, Wandering Eye, etc.
3 Simple Fighter Used by Zombies, Goblins, Undead Miners, Crabs, etc.
4 Eye of Cthulhu Used by the Eye of Cthulhu and Ocram (Console version) bosses.
5 Aggressive Flier Used by Big Eater, Hornet, Meteor Head, Destroyer Probes, Eater of Souls, etc.
6 Burrower Used by Bone Serpent, Devourer, etc.
7 Friendly NPC Used by town NPCs, Bunnies, etc.
8 Teleporting Spellcaster Used by Tim, Imp, Goblin Sorcerer, etc.
9 Magical Projectiles Used by Burning Sphere, Chaos Ball, Water Sphere.
10 Hesitant Flier Used by Cursed Skull.
11 Flying Skull Used by Dungeon Guardian, Skeletron's Head.
12 Flying Arms Used by Skeletron's Arms.
13 Carnivorous Plant Used by Man Eater, Snatcher.
14 General Flier Used by Cave Bat, Demon, Harpy, Jungle Bat, etc.
15 King Slime Used by the boss King Slime.
16 Fish Used by Piranha, Shark, Goldfish, etc.
17 Vulture Used by Vulture.
18 Jellyfish Used by Pink and Blue Jellyfish.
19 Antlion Used by Antlion.
20 Spike Ball Used by the Spike Ball found in the dungeon.
21 Blazing Wheel Used by the Blazing Wheel found in the dungeon.
22 Hovering Fliers Used by Wraiths, Pixies, and Gastropods.
23 Animated Weapons Used by Enchanted Swords and Cursed Hammers.
24 Birds Used by Bird.
25 Mimic Used by Mimic.
26 Unicorn Used by Unicorn.
27 Wall of Flesh (Mouth) Used by the boss Wall of Flesh's mouth.
28 Wall of Flesh (Eye) Used by the boss Wall of Flesh's eyes.
29 The Hungry Used by The Hungry, spawned from the Wall of Flesh.
30 Retinazer Used by the boss Retinazer.
31 Spazmatism Used by the boss Spazmatism.
32 Skeletron Prime (Head) Used by Skeletron Prime.
33 Skeletron Prime (Saw) Used by Skeletron Prime.
34 Skeletron Prime (Vice) Used by Skeletron Prime.
35 Skeletron Prime (Cannon) Used by Skeletron Prime.
36 Skeletron Prime (Laser) Used by Skeletron Prime.
37 Destroyer Used by the boss The Destroyer.
38 Snowman Used by Snowman Gangsta, Mister Stabby, Snow Balla

you can specify the ai by using 'aiPretendType = (int (npc type)) ' to mimic a specified NPC