>>718722623
It's not a head, but there's a few frames in this where Tokiyuki has extra arms. It's an exaggeration of the effect where something moving really fast appears to smear and be in more than one place at once. So a character shaking their head might be animated like that.
Extra fingers would be for something like a character shaking their hand after touching something hot. In this case Wolf is rolling forward and there's not much motion in his hand, so it wouldn't be appropriate here. But >>718717649 is most likely correct and it isn't a finger at all, "hallucinated" or deliberate.