Tam O' Shanter

The Tam O' Shanter is a vanity item in form of a bonnet. It is sold by the Clothier in Hard mode two nights after a full moon.

Trivia
The name "Tam O' Shanter" stems from the poem by Robert Burns with the same title where Tam, the protagonist, is wearing such a bonnet. It also may be a refrence to the hat from Team Fortress 2.