This new addition to the popular Do You Know series tells young naturalists everything they ever wanted to know about rhinos. Do you know? The record for the longest horn is held by a white rhinoceros. Its horn was 1.66 metres (5.2 feet) long. A broken horn will regrow in two to three years. Just like our nails and hair, a rhinoceros horn is made of keratin. The rhinoceros is an endangered species. There are only about 8,000 individuals left in the world.