This new addition to the popular Do You Know series tells young naturalists everything they ever wanted to know about rhinos: — 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.