Horns, Antlers, and Beyond

In my recent post about giraffes, I forgot to cover an important part of their wonderfully bizarre appearance – those “horns” on their head. I say “horns” because they’re not technically horns – they’re called ossicones. When I found that out, it got me wondering: how are ossicones different from horns? And are horns different from antlers? Why do animals have horns or antlers or whatever at all?