The One and only Ivan by K.A. Applegate

Ivan is a gorilla who lives in a tourist trap mall off I95.  He has friends, Stella, an elephant, Bob a dog, and Julia, the janitor’s daughter, who brings him materials to do art.  Maurice, who owns the mall and the animals in it, raised Ivan after hunters killed his family.  But Stella is sick and Maurice doesn’t take care of her as he should.  A baby elephant, Ruby, captured as Ivan was, comes to the mall.  She’s scared.  Stella gives her some comfort and pointers, but Stella dies and just before she does, she asks Ivan to take care of the baby.  Ivan has to think for a long while, but he starts painting pieces of paper and makes enough to cover the billboard.  Julia talks her father into putting the pieces together on the billboard and suddenly the mall is the center of attraction again.  Maurice is happy at first to be making so much money.  But eventually the zoo learns of the animals and comes to inspect their cages.  In the end, Ivan and Ruby get to move to an open-air zoo.