Tier I: Comepletely Passive Mobs Who Wont Fight Back In Any Circumstance.
Tier: I/(1.5): Mobs Who Are Passive, But Will Fight Back In Certain Circumstances. Such As Being Hit, Or Not Wearing Gold Armor.
Tier II: Normal Sized Hostile Mobs.
Tier II/(2.5): Large And/Or Strong Hostile Mobs Who Aren't Sub-Bosses.
Tier III: Sub-Boss Mobs Who Are Stuck In A Specific Area.
(Still considered Tier III)
Tier III/(3.5): Sub-Bosses Who Are Either Roaming, Can Leave Their Specified Area, Or Only Becomes Hostile In Certain Circumstances.
Tier IV: Boss Mobs
Tier V: Mega/Rare Bosses Who Are Extremely Strong, Who Have Special Drops.
Tier VI: Special Mobs Who May Or May Not Be Hostile, They Often Have Special, Unique Abilties, Or The Only Entity Of Their Kind That Can Spawn In A World