There are three main symbols in Eugene O'Neill's play "The Hairy Ape."
One symbol which is predominant is the figure of the ape itself. Not only is the word ape used in the title (which is a clue to its relevancy), apes are everywhere (or referred to many times) throughout the play. Yank acts like an ape and is constantly compared to an ape.
Another symbol in the play is Rodin's statue "The Thinker." This symbol is important because Yank constantly worries about thinking. He desires the ability to have the ability to think in the way that the statue does (or how he thinks the statue thinks). Yank uses the pose of "The Thinker" to show his struggle with thinking and his obsession with the practice of thinking.
The last symbol in the play is the use of steel. Steel can be considered in two very different ways. First, steel is strong and powerful. It is when he is placed in challenging situations where he imagines his strength being like steel. Contrastingly, steel is also an image of oppression. Cages are made out of steel, and the bars of a steel prison are inescapable. Both Yank's cage and the cage of the apes are made out of steel.