Not just the nostril, but also the pointed lip of the 'black' rhino which differentiates it from its 'white' relative.
It is commonly stated as a 'stock animal fact' that the white rhino's name comes from a corruption of Dutch 'wijde', meaning 'white'. However, this factoid has fallen into debate in recent years as older hunting references seem to call this species not as the white rhinoceros, but as the Burchell's, or Flat-nosed Rhinoceros. The first mention of a 'white' rhinoceros came in 1801.. made in reference to a *black* rhinoceros. And even then surprise was noted at how these rhinoceroses were called as 'white' when they were ashy grey.. Others have tried to have their own explanations for why this name came to be.. but there is no conclusive answer as of yet.
But this does make one wonder... how did the black rhinoceros get its name?
