Leonard was first seen in the first episode, "Born to Be Beavers". He appeared throughout the series after he kicked his sons out of the house.
Leonard is a normal American beaver with blond fur (with a crew cut), a monobrow, a cleft chin, and a light purple nose. He wears black-rimmed glasses and and a watch on his left wrist.
In the episode "A Little Dad'll Do You", he wore a purple shirt with white-and-green flowers and a light blue collar and cuffs and also blue and light blue striped pants. Under that, is a white sleeveless shirt and lime green shorts.
The Angry Beavers Wiki has a gallery for Leonard Beaver