sorry? elephants have the same, and they are herbivores

I agree with Xav.The horns might of haved been used to protect themselves from other carnivorous creatures.

Or it could be used to impress the females. The bigger the tusk, the fiercer it looks, and safer the female is going to feel, or the safer she thinks the young will be, and so is programmed to be more attracted to that.

Or it could be a redundant feature.