In Norse mythology, Hildisvíni (“battle swine”) is Freyja's boar which she rides when not using her cat-drawn chariot.
It is also said to be Freyja's human lover, Ottar, in disguise, and is the reason why Loki consistently accuses her of being "wanton" by riding her lover in public.
It was also the helmet of Áli that was taken by the Swedish king Adils after the Battle on the Ice of Lake Vänern that ended with Áli's death.
In Beowulf, it is said that a boar on the helmet was there to guard the life of the warrior wearing it.