This is the best information we could find on the tier drop system. If it is inaccurate please let us know. Tier 1: Tortus, Poultry, and Pantera. Drop at rappelz-item.com/tag/level/">level 1 mobs and above.
Tier 2: Orc, Siren, Blue Pixie, Red Pixie, Yeti, Skelly, and Wolf. Start dropping from level 30 mobs and above.
Tier 3: Hawkman, Sallys, and Harpies. Start dropping from level 90 mobs and above.
Tier 4: Angels, Kentas, Cerbs, and Gnolls. Start dropping from level 120 mobs and above. However, empty cerbs only drop from Cerberus mobs and one other foe.
Tier 5: White Dragon. Empty White dragons only drop from other White Dragons and one other foe.