Bran's direwolf, Summer, was among the list of casualties in the fifth episode. He sacrificed himself to protect Bran, Meera, and Hodor from an incoming group of wights. Though the death of yet another Stark direwolf was hard to swallow, the metaphor made sense. 

Winter has officially come to Westeros, and the White Walkers came with it. Summer, both the season and the direwolf, were killed by the onslaught of winter.