Mrs. Beaver was first seen in the first episode, "Born to Be Beavers". She appeared throughout the series after she kicked her sons out of the house.
Mrs. Beaver is a normal American beaver with brown fur and hair, light brown eyes, and a red nose. She wears a pink pearl necklace and earrings. In "A Little Dad'll Do You", she wore a light blue apron while working in the kitchen at the Beaver family dam.
She picks her toe, snores when she's sleeping, and flosses her teeth with a shoelace that Leonard gave her on their first date. 
The image gallery for Mrs. Beaver may be viewed here.