The Withernosaurus is a strange and legendary beast that Olly reads about in a book called Legends of Withern Mere.  It was said to have  been  dropped  accidentally  into the bottom of Withern Mere as an egg, from the claws of its parent flying overhead, and to have hatched out there in the murky waters under some boulders.  For the next seven thousand years it stayed there, growing, until it first surfaced and was  spotted  by  some men  out  hunting with  stone axes  for  weapons  and  deerskins  for  clothes. 

According to Olly's book, the Withernosaurus  was several  hundred  feet  long, with sharp pointed fins.  Its scaly body was slime green, its eyes were yellow and it had thousands of sharp teeth.