In the book Maniac Magee, in which pen does Jeffrey sleep?

Jeffrey sleeps in the buffalo pen at the zoo.

As Chapter 22 begins, we learn that Jeffrey has been spending his nights with the mother and baby buffalo in their pen at the Elmwood Park Zoo. Each morning, he hops back over the fence to spend the daylight hours finding food; at night, he hops the fence again to nestle in with those animals.

However, despite this workable living arrangement, Jeffrey isn't doing too well. Presumably out of weakness caused by undernourishment, he falls one morning while trying to climb over the fence, remaining there just outside the pen in an unconscious clump until the kind old man Grayson discovers him.

At that point, Jeffrey is thin, scratched, and dirty. The buffalo pen has been a safe place for him, and he's enjoyed bonding with the baby buffalo, but as a result Jeffrey smells awful and desperately needs a bath.

Toward the end of Chapter 22, we find out that he had originally tried to sneak into the deer pen instead of the buffalo pen, which makes sense considering how Jeffrey and Amanda had previously talked about that as a possible place Jeffrey could stay. But as Grayson explains:

"They switched the deer and the buffaloes around last month."

So Jeffrey stays with the buffalo—an interesting choice, one that shows how adaptable Jeffrey is to the ever-changing situation of living as a homeless child.