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Filthomena is a grouch who attends Oscar's Grouch Reunion in the 1989 book What's in Oscar's Trash Can? and Other Good-Night Stories. She swims in Oscar's pool with Grungetta, Oswaldo and Fluffy the Elephant.

Filthomena also plays in Oscar's Junk Band in the 1994 book Oscar's Grouch Jamboree.

In another 1989 book, When Oscar Was a Little Grouch, Oscar babysits his young niece, who is also named Filthomena.