As a title, The One and Only Ivan works on a couple of levels. First off, it's a nickname Mack gives our main ape as a means of drawing people into his depressing mall circus. It's also a nickname that Bob coopts for Ivan, though, to build his self-esteem and help him see himself as the superstar gorilla that he is.
On a more symbolic level, though, the title is a reference to Ivan's solitary existence. He spends so much of his life in an indoor cage with no opportunity to interact with his species or learn from the behavior of other gorillas. He is, for nearly three decades, the one and only gorilla he knows; he even stops thinking there are others in the world at a point.
Lastly, the title is a bit of endearment. Ivan is the one and only gorilla in this story, and as such, he really steals readers' hearts. It doesn't take long before he feels like our one and only instead of just some random gorilla.