Misery Moo by Jeanne Willis, illustrated by Tony Ross

From chapters.ca:
Introducing the funniest and gloomiest literary character since Eeyore, Jeanne Willis and Tony Ross have created a wonderful story for young readers about the importance of friendship and being a friend, even when it seems impossible! Misery Moo is constantly looking at the downside of things, while her friend Lamby Poo goes through life wearing rose- colored glasses, and trying to get Moo to see things the way he does. However, Misery Moo's pessimism soon gets the best of Lamby Poo, and it is up to this miserable old cow to make her friend happy again.