Abandoned Tunnels

You look back to the beast. It's looking at you affectionately. It seems more intelligent than you first assumed. You walk past it further into the jungle. It rushes around you and stops in front of you. Then it picks you back up and sets you at the edge of the jungle. You walk past it again. This time, it simply follows you. 

After a while of walking aimlessly through the jungle, the beast huffs and picks you up again. It brings you to a nest, of sorts. Then it leaves.

A while later, it returns with some strange fruit and a few corpses of other mutated creatures. It offers you the fruit, then sits down to eat its prey.

You live like this for a few weeks. Exploring the jungle with your new friend, hunting, foraging, and living in peace. Eventually, you graduate from being carried to riding the creature. The two of you form an understanding, and a deep bond. 

Using scraps of metal from the walls, you make tools, and spend months building yourself your own home woven from various vines and leaves. 

You live out the rest of your days in the jungle. It's not always an easy life, but you never again face the constant stress and fast-paced lifestyle of rodent society. You may not interact with other rodents anymore, but you're never alone. The beast is always by your side, ready to protect you, hunt for you, and snuggle with you when it gets cold.