There's a good deal of animal imagery in Never Let Me Go. One has only to think of Tommy's elephant drawing or his drawings of imaginary animals at the Cottages. Then there's Kathy's comparison of herself and her friends to spiders when they test to see if Madame is scared of them.
Animal imagery is generally used throughout the book to highlight the creativity of the clones. Students at Hailsham express themselves, creatively mainly through the painting and sculpting of animals. But then, that's not really surprising when one considers that, as clones, they're treated like animals themselves. It's as if they instinctively empathize with the condition of animals in a human world, given that they too are subject to the kind of exploitation meted out to members of the animal kingdom.
Despite the profusion of animal imagery in the story, there are no actual real-life animals on display. This highlights the fact that, for clones, animals exist largely in the imagination. On the face of it, the clones might look and behave just like regular humans. But in reality they're completely different, with an inner life unlike that of humans. And because of this they have a radically different attitude towards animals.