MoneyDrop

From MothCraft

MoneyDrop drops virtual money, represented by an ingame item, when a mob or player dies. This money can be picked up by players, but will go into their wallet instead of their inventory.

These are "Gold Nugget" items you find in mob loot after killing a mob or player. Each mob has a different price. most Peaceful mobs will not drop money simply to avoid infinite money farms.