From the Super Mario Wiki
Bone Goombas are enemies that appear in New Super Mario Bros. 2. Appearing alongside other bone-related enemies like Bone Piranha Plants and Dry Bones in Towers and Castles, they wear skulls on their heads, but other than their appearance, they seem to be identical to ordinary Goombas, slowly walking forward and being able to be defeated with any attack.
When defeated, the skulls on their heads are smashed, revealing that they have a completely blue body similar to the Gloombas from the Paper Mario series and the Goombas found in caves in Super Mario Bros.. Additionally, they appear without skulls in World Flower-, World 6's Ghost House and in the Gold Classics Pack.
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