*disclaimer: actually, giraffes uses their skull as a flail, of which their necks are used to swing during the mating/superiority battles between males (known as necking). That's the reason for those two strange bulb-shaped horns.
And no, while their neck bones looks tough enough to withstand impact and battle it remains a vital structure for their survival, so no sane giraffes will ACTUALLY use their neck in battle as club.
