Thunder Mist is the name of a wolf Perrin comes briefly into contact with. He or she is a member of Leafhunter's pack.
While in Altara, Perrin has a dream in which he smells something worse than a Myrddraal (Darkhounds) and contacts Leafhunter's pack of wolves to ask what it is; he cannot remember the exact scent, and so asks instead what wolves hate more than Myrddraal, something that a wolf knows means death. Perrin can feel the wolves' anger and hatred at the mention of the Darkhounds. Thunder Mist and all of the others, with the exception of Leafhunter, break contact with Perrin, and Perrin thinks that these wolves will not speak with him again any time soon.
(Reference: Crossroads of Twilight, Chapter 5)