The boar was a totem animal for certain Celtic tribes going into battle and was worn on their helmets, because it was considered to be fearsome and hugely aggressive, so they emulated the strength and fervour of this magnificent beast. Sacred to the god Cernunnos and the Goddesses Epona and Rhiannon.

For more background information and exposition of the boar's role in Celtic culture, see the following webpage:
http://www.writer2001.com/boars.htm