Here's a list of backpacks only dropped by creatures and not obtained by a NPC, quest, or reward:
Buggy Backpack dropped by Kollos
Book Backpack dropped by Faceless Bane
Ghost Backpack dropped by The Pale Worm
Festive Backpack dropped by Piñata Dragon
Winged Backpack dropped by Urmahlullu the Weakened
Old and Used Backpack dropped by a specific Orc
Pillow Backpack dropped by Lloyd
Feedbag dropped by
Deepling Backpack dropped by
Mushroom Backpack dropped by
Jewelled Backpack dropped by
was also a replacement of old key rings.... so that was another way to obtain it when they made that change
Wolf Backpack dropped by
Source: https://tibia.fandom.com/wiki/Containers
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