I'm not sure if this is all possible, but here are my ideas. I built these assuming the gamemode is either survival or DD, and not necessarily creative.
Wanders around a tree. Becomes aggressive towards any other non-Djinn mobs that get close to it.
You mean the Dryad. The Djinn are more or less Arabic demons. Dryads are tree sprites.
To add on to this, I believe Dryads should be neutral to players until they see a player either A cut down a tree, Or B attack another dryad.
Alright, Here we go.
Diablo: Wears Titanium Leggings, Gauntlets, And boots. (To reflect the avatar.) Spawns randomly with either their fists, A titanium sword, Or a Titanium Warhammer.
Diablos are smart and have the ability to pathfind around obstacles and traps.
When fought alone: A Diablo will strafe you whilst attacking you, When it's health gets below 25% it will attempt to retreat and regenerate. Diablos will receive an accelerated regeneration bonus when in lava.
When fought with other Diablos: When fighting multiple Diablos, They try to surround you to the best degree possible. Acting basically the same as solo combat other wise.
When fought with other mobs: When the Diablo sees another mob whilst in combat with you, It will call them into a formation dependant on the mob (See When rallied by Diablo.) and retreat to the back of the formation until all other mobs are killed.
The idea is that Diablos should be a sort of terrifying field general that makes mobs unite in formation to murder the player.
Orc: Spawns with iron armour, A steel shield, And a steel spear.
Orcs are rather stupid, They have a basic idea of how to avoid things that would get them killed. But in the heat of combat they may accidentally fall to their demise.
When fought alone: Orcs will play a more defensive role, Holding up their shield and waiting for you to move in close to attack.
When rallied by a Diablo: Orcs rallied by a Diablo will assume a phalanx formation. They synchronize their attacks so that large chunks of damage are done. When one of the front lines is killed, The orc behind it moves forward.
Orcs make good fodder, They are not weak, But they aren't strong. A good front line soldier that won't die immediately.
Feel free to make more militarized formation mobs, Ideally Diablos could rally up most mobs. I just can't think of many that would be a threat.
Spider: I'm assuming our spiders are like jumping spiders. Regardless, They need to be majorly buffed if this behaviour is to be added.
Spiders would run a check on their target to see if they could reasonably kill them. If so, They are labeled as prey, If not they are labeled as a predator.
A spider will attempt to stay behind it's prey, Slowly walking behind it until it gets within about 14 blocks away, At which point it will jump at them doing a large amount of damage if it hits.
If the spider can not kill the player within a few hits, It jumps about 25 blocks away (This is lower than realistic distance for a spider like that.) and continues to stalk them for another jumping attack.
This goes on until something dies.
If a spider labels something as a predator, It will flee if it gets closer.
Spiders would hunt and attack anything within a reasonable level of it.
Hermes Wraith: To me, The Wraith feels like it would be a great necromancer, It would attack you by launching fireballs from it's staff at long range, And hit you with it's sword at close range.
But the main danger of the wraith is that it summons zombies and skeletons which will not die until the wraith is killed.
Every 30 seconds the wraith will revive all dead minions back to 50% HP.
When the wraith dies, All summoned minions are killed.
Skeleton: The skeleton could spawn with any weapons and armour in the game, And is rather weak on it's own.
A ranged skeleton will stick back behind other mobs, And attack with it's weapon.
A melee skeleton will run forwards and attack with whatever weapon it has.
All skeletons in the world by default are summoned by necromancer wraiths.
When near a wraith, They can not die. Rather they are stunned when their HP reaches 0 and they will revive to 50% HP when the wraith uses it's revive ability.
Zombie: Spawns with nothing.
When a zombie attacks you, You have a chance to be poisoned. Draining HP at a rather slow rate until it wears off.
Zombies are really dumb, They cannot navigate the simplest of obstacles.
When near a wraith, They cannot die. Rather they are stunned when their HP reaches 0 and they will revive to 50% HP when the wraith uses it's revive ability.
Original: When you attack it, It becomes invincible until it either tries to attack you, Heals, Or moves.
Likewise, When it tries to attack you, It then waits to attack you until you either swing an item, Or move.
This is meant to sort of simulate a turn based combat system.