Cow and Chicken - Season 2 Episode 11 -

The Red Guy sneaks a crow egg under Chicken's covers while he sleeps. The next morning, Cow and Chicken find the egg, and Chicken comes to the conclusion that he is actually a girl. So, it's egg sitting for Cow and Chicken, until the egg hatches and a baby crow comes out. Now Chicken comes to the conclusion that he isn't even a chicken.