horn

Preferred label: 

Outgrowth of modified skin or compacted hair, often occuring in pairs, projecting from the frontal bones on either side of the skull, consisting of a nondeciduous cuticle composed of keratin. Distinguished from "antler," which is modified bone.

AAT

The Arts and Architecture Thesaurus Online, The Getty Research Institute.

Additional reference: 
http://vocab.getty.edu/aat/300011826
Hierarchy: 
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Top concept: 
materials