Not bills. Loonies are dollar coins with a loon on the back. "Twonies" are the two-dollar coins with a polar bear. Canada traditionally uses animal imagery on money, a sort of reconciliation gesture given the french-english-native splits in the country. Animals were common ground.

Fyi, the band Nickelback was Canadian. The non-wikipedia joke behind that name is the animal on the back of the canadian nickel.