The Item Bosses are special bosses that only appear every 10 floors. They include the Item General, Item King, and the Item God. Rank 40 items also have a special boss known as the Item God 2. A Gatekeeper is also noted here, though they are more of a sub-boss of the item world.
Despite all other enemies being able to be thrown on a Geo Symbol in order to destroy it in Disgaea: Hour of Darkness, Item Bosses can not be thrown on a Geo-symbol. The game will simply buzz the player.
Item Generals
Item Generals are seen on floors 10, 20, 40, 50, 70, and 80 of the item world. They'll often be seen standing atop a plateau of sorts on that particular floor. They also will not move from the plateau until a player's character moves into its attack range, unlike all the other enemies in the item world, which are drawn to you if they are able to move and attack the player's characters.
Item Generals can be any type of class and tier, picked at random (save for the storyline characters and their classes). They often have more HP and higher stats than the class/tier they are would normally have.
Item Generals offer a 1.5 times experience and hell bonus for defeating them. The experience and hell they are worth is entirely dependent on what class and tier they are.
Also, when they are defeated and a player exits the item world, Item generals give a much larger stat boost than normal to the item being leveled, and any stat that is not 0 (positive or negative) gets a boost from a defeated item general.
Note: A bug was noted in Disgaea: Hour of Darkness concerning Item Generals. If a player defeated an Item General and cleared the level, then chose to continue on in the item world (rather than return to the Overlord's castle), then exited at any point after that, the Item General kill would not be counted. This bug was fixed in both remakes, Disgaea: Afternoon of Darkness and Disgaea DS.
Note2: It's possible for an Item General to be absent from the mentioned floors, if a Geo Symbol occupies the panel it would normally spawn. (Disgaea: Hour of Darkness)
Item King
Item Kings are seen on floors 30, 60, and 90 of the item world. They're often seen standing atop a plateau of sorts on that particular floor, although their plateau tends to be less steep than an Item General's. They also will not move until a player's character moves into its attack range, unlike all the other enemies in the item world.
Item Kings can be any type of class and tier, picked at random (save for the storyline characters and their classes). They have far more HP and even higher stats than the class/tier they are would normally have.
Item Kings offer a 2 times experience and hell bonus for defeating them. The experience and hell they are worth is entirely dependent on what class and tier they are.
Also, when an item King is defeated and a player exits the item worlds, Item Kings give a much larger stat boost than normal to the item being leveled, and any stat that is not 0 (positive or negative) gets a boost from a defeated item king.
What's more is a defeated item king will increase the Specialist capacity of the item being leveled by 1.
Note: A bug was noted in Disgaea: Hour of Darkness concerning Item Kings. If a player defeated an Item King and cleared the level, then chose to continue on in the item world (rather than return to the Overlord's castle), then exited at any point after that, the Item King kill would not be counted. This bug was fixed in both remakes, Disgaea: Afternoon of Darkness and Disgaea DS.
Gatekeeper
A 'Gatekeeper' could be considered a sub-boss of the item world. They appear randomly, but they never appear on floors where special bosses appear (see above) Gatekeepers do not move, enemies on dimensional gate cannot be Lifted or thrown, and spawns sitting on the Red Portal to the next world, blocking anyone who tries to enter it (sometimes regular enemies spawn at dimensional gate (they can move)). The best way to handle a gatekeeper is to either attack it from out of its attacking range (as it cannot move anyway), or simply moving the gatekeeper via an attack that moves the target (many Fist special attacks are best for this, such as Triple Strike), or warping him out of gate (he still can't move).
Gatekeepers do not offer any sort of special Hell or Experience bonuses, and their stats are like a normal enemy's stats. (of whatever class and tier they happened to be assigned)