Deleuze says, Literature stands for 'becoming-animal'. Literature also represents 'becoming-woman' and 'becoming-molecule.' What does 'becoming-animal' mean? Is it the traditional way of representing 'animality' as something which lacks restraint to biological impulses. Of course not. I understand 'becoming-animal' as something which is incapable of symbolizing anything other than what is present. For me 'animality' is incapable for symbolization and hence which is in presence always. Animal accepts what has presence and that which is not debated for being absent.
One may ask, why has Deleuze chosen only Literature among the multitude of texts which also deserves attention. Why didn't he say that those feminist writings are more like 'becoming-woman' than their less-hyped counterparts? My understanding goes like this, Deleuze's statement appears to be problematic for us when we first try to define 'woman' and then look for correspondences in literature. The pitfalls of such traditional methods of literary criticism are quite predictable; they configure texts as Feminist, Marxist or the like. Instead it should be through the activity of literature that a woman should be born. Alternatively if we try to symbolize woman, it becomes exactly the opposite of literature.
Literature is also 'becoming-animal' in the sense that it is beyond representation. For me, the 'becoming-animal' character of literature primarily has a footing on the non-judgemental nature of it.
A survey on Deleuze's philosophy: