A thick-skinned monster is a monster that will only be dealt one hit point damage when attacked by a bullwhip. Thick-skinned monsters have the flag M1_THICK_HIDE in the source code.

The following monsters are thick-skinned:

• Gargoyles and winged gargoyles
• All mimics
• Mumaks, titanotheres, baluchitheriums, and mastodons
• All dragons (including the Chromatic Dragon and Ixoth)
• Earth elementals
• All nagas
• Xorns
• Skeletons
• Wood, clay, stone, glass, gold, and iron golems
• Horned devils and barbed devils
• Sharks
• Crocodiles and salamanders