The first thing you need to know is she had the coolest name of all the dogs—Cookie. (No, not that Cookie.)
Secondly, Cookie was an Alaskan sled dog. She makes only a brief appearance in the book, but she's a hugely important figure in Paulsen's life. "She was a good friend, a kind of dogsister or dogmother" in his eyes (1.3). Oh, and she also totally saved his life.
For the most part, My Life in Dog Years is a series of portraits; each chapter tells you the story of one dog's life. Cookie's chapter is different because it's a dedication (the book itself is dedicated to her memory), not a portrait. Pretty much the only thing we know about her life story is that she came to Paulsen very sick with worms and eventually became his lead dog as he sledded around Alaska. "I have written much of [Cookie] in other places," Paulsen says, but "she belongs in this book, too" (1.4).
Instead of giving us her biography, Paulsen focuses on the time Cookie saved his life in 1980. Long story short, he fell through thin ice into cold, cold water when the temperature was 50 below.
Way to go, slick.
Cookie rallied her team and pulled him out of the water, saving his life.
She saw me drop, instantly analyzed the situation, got the team up—she must have jerked them to their feet—got them pulling, and they pulled me out. (1.17)
Her speed and competence saved him from drowning or freezing to death. Probably both.
Everything that has happened in the last seventeen years—everything: Iditarods, published books, love, living, life—all of it, including this book, I owe to Cookie. (1.18)
We don't know the details of her life, or even the basics of her personality. What we do know is that Paulsen's love, admiration, and gratitude for this dog is off the charts.