The horn of a rhinoceros is made of keratin – the same protein found in human hair and nails.
A horn is a structure made of keratin borne on the head of many ungulates. Horns come in a variety of forms. They are usually permanent structures though the antlers of many deer are cast and regrown annually. Horns appear occasionally to be purely ornamental, but usually they are used for defense or intra-specific aggression. In such species horns are borne only by the males.