Ever heard the phrase, "He's got a memory like an elephant?" Turns out that's not just a colloquial way of speaking. Elephants are some of the smartest animals on earth, and they "have excellent memories" (source). Rosie is no exception. She doesn't forget kindness, and she certainly doesn't forget cruelty, which August finds out the hard way.
As Jacob emphasizes, an elephant can seem almost as with it as a human. At one point, after August abuses her, "[Rosie] stares at [Jacob], a look of unspeakable sadness on her face. Her amber eyes are filled with tears" (20.113). Rosie has real feelings and can even cry. In many ways, she is one of the most "human" characters in the book. (Check out Rosie's "Character Analysis" for more on this.)
So, if an animal can be human, that probably means that humans can be animals, too, right? We see violence and brutality galore in our human characters, and Rosie's presence is a great reminder of this.
Elephants also come to symbolize the idea of the circus in the book. (For more on this idea, head over to "What's Up With the Title?".) McGuinty uses the idea of watering them to prove he used to work for a circus. But that statement instantly tells Jacob that McGuinty did no such thing. As he says to Rosemary, "Pffffft. Carried water for the elephants indeed. Do you have any idea how much an elephant drinks?" (1.59).
While this is meant as a put-down of McGuinty, it's also a good reminder of just how big elephants are. If you check out the "Afterword" to the novel, you'll find stories about elephants that turned on people and murdered them. An elephant may not appear as dangerous as a carnivore like a lion or a panther, but it's still much stronger than a human. And yet, as strong as Rosie is, for much of the book she can't protect herself against someone as cruel as August.
This vulnerability is a trait Rosie shares with other characters, specifically Marlena and Jacob. Like Marlena, Rosie performs for others and is abused by August. Both are treated like scapegoats. Like Jacob, Rosie is a kind of outsider who falls into circus life by accident and almost doesn't succeed. Ultimately, though, Rosie does what neither Marlena nor Jacob can bring themselves to: she kills August. And unlike her human counterparts, she doesn't seem to lose any sleep over the murder. She is just an elephant, after all.