The dead mouse in Lennie's pocket has a number of symbolic meanings. Firstly, the fact that Lennie finds it already dead is important because it suggests that his and George's move to a new ranch will not offer the fresh start that they are looking for. This foreshadows the death of Curley's wife and the trouble which follows.
Moreover, carrying the dead mouse around is symbolic of Lennie's inability to escape the past. The incident in Weed for example, will haunt Lennie and George, no matter how hard they try to put the past behind them, and not everybody will see that it was an accident.
In addition, the dead mouse is also symbolic of the dangers of Lennie's strength. Although he has a gentle and caring manner, Lennie lacks the ability to control his strength. Whether it's a mouse, a rabbit, or Curley's wife, his desire to stroke and pet soft things always leads to death. It does not matter that these deaths are accidental—what matters is that Lennie's strength will always injure those that he cares about.
The dead mouse that Lennie carries in his pocket at the beginning of the novel symbolizes Lennie's vulnerability. Similar to how the mouse is physically small and defenseless, Lennie's mental handicap leaves him helpless in a harsh and cruel world. The mouse cannot withstand Lennie's strength and dies because Lennie pets it too hard. As mentioned in the previous post, the defenseless mouse also represents Curley's wife. When Lennie grips onto her hair, she tries to escape but cannot. Similar to the dead mouse, Curley's wife is also accidently killed by Lennie. In spite of Lennie's innocence, he cannot help himself from inadvertently harming things. Alluding to Robert Burns' poem, neither mice nor men can escape their destiny. The dead mouse symbolizes how innocent creatures such as Lennie and Curley's wife cannot escape their fate.
Lennie carries a mouse in his pocket for comfort. He likes to "pet soft things" and is soothed by the feel of fur. It is perhaps linked to his memories of his Aunt Clara who used to give him pieces of velvet to stroke.
Lennie always accidentally kills the mice he has, which foreshadows his final action of killing Curley’s wife. The mouse represents frailty, weakness and comfort, all of which we see in Curley’s wife, and part of the charm which captivates Lennie.
The use of the mouse also ties in with the title taken from the Robert Burns Poem alluding to the fact that the best laid plans of mice and men often go wrong.
Lennie, we quickly realise, is huge and unaware of his own strength, which frequently gets himself into trouble. He loves animals. The mouse may be a symbol of his simplicity and his otherness. After all, few grown men carry mice in their pockets. We learn that he constantly kills the mice he so loves, by literally "loving them to death" and this angers George.
The mouse image or symbol also introduces the theme which becomes the central one of the book. The inadvertent killing of the mice escalates into the killing, however, unwittingly of other larger animals such as the pup and later of course of the woman, Curly's wife, in the barn.