If you own a Obsidian Knife, "use with" on a monster body Leather can be obtained as loot from monsters as well, or even be found as quest rewards. Be aware that it will not always work the skinning attempt, and you also will be able to skin a monster only once, doesn't matter if it fails or succeed. But it is not very monster that can be skinned. Here is a list of monsters that can be skinned:
Obtained from: https://tibia.fandom.com/wiki/Skinning
You can obtain an Obsidian Knife buying in the Market or by completing the Obsidian Knife Quest. This quest can be done by a Free Account Player.
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