I think it was discussed before, but recently I was impressed again by horns of some wild rhinos. Not only much longer than in zoos, but also different shape - narrow and pointed. Zoo rhinos have more like thick stumps...
The rhino horn trade is now legal in South Africa, as the last few days saw an overruling of a ban placed in 2009. Some sources to read about it here:
Rhino horn trade = extinction in the wild - Africa Geographic
South Africa to Legalize Domestic Rhino Horn Trade | Smart News |...