Mack-truck sized squirrel, named Ratatosk.
If that doesn't get your attention, I don't know what will. This was the best yet in The Iron Druid Chronicles. I was constantly worried about one thing or another - like will Atticus ever get his clothes back. Like before the next fight.
Luke Daniels did a great job as the narrator, even as a squirrel.
This is one of the funnest urban fantasy series out there.