The New Year's Eve Tiara is an item in Nintendogs that falls under the category of "Accessories" in the Supplies. It is a golden tiara with the words "Happy New Year" on the front of it and can be found during walks. It sells for $12 at the Secondhand Shop.
DS: "Ring in the new year with the perfect headgear for your own party animal."
It is one of the accessories to not be available in the original Japanese version of Nintendogs.