keratin

https://doi.org/10.1351/goldbook.10873
Fibrous protein forming the main structural constituent of hair, feathers, hoofs, claws, horns, etc.
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 771 [Terms] [Paper]