The Wild Pack by Andre Marx, Boris Pfeiffer

Hamlet, a spirited young wolf, escapes the zoo to search for the Wild Pack—a band of animals living in abandoned rail tunnels and caves under the city. They have only one goal: to be free once again. But instead of the bold animals that he was expecting to encounter, Hamlet finds a scraggy, ragtag bunch. Will he be able to motivate the animals to help him rescue his friend, the gorilla, from the zoo? "