Keratin is a protein that comprises rhinoceros horns, human hair and fingernails, and animal hooves!
Rhinos were nearly hunted to extinction because of the perceived medicinal powers of their horns. Pretty sad when you consider that their horns aren't even made of a rare material.
As of right now, there are only 275 Sumatran Rhinos left, 40 Javan Rhinos, and only 4 living members of the Northern White Rhino subspecies.
For more on rhinos.