What are the symbols found both in the title Of Mice and Men and in the novella?

In Of Mice and Men, the symbolism of its title is twofold.

First, the title represents the fact that the plans we make can often fall apart. Steinbeck took the title from Robert Burns' poem "To a Mouse Upon Turning Up Her Nest With a Plow." In this poem, the speaker apologizes to the mouse for destroying her home right before the frosty winter. He acknowledges that the mouse probably planned and worked very hard to prepare for the upcoming freeze, but he also states that "the best laid schemes of mice and men often go awry," meaning that no matter how much we try to plan and prepare, something can always go wrong. Such is the case for George and Lennie who devise a plan to save their earnings and buy a farm where they could live the rest of their days "an' live of the fatta the lan" (14). However, as the poem states, their plan falls apart when unforeseen circumstances lead to Lennie's death.

Lennie and George's farm is a symbol itself, representing the unattainable American Dream. According to Crooks, "every damn one of 'em's got a little piece of land in his head. An' never a God damn one of 'em ever gets it" (74).  

The mouse in the title also symbolizes the power that the weak have over the strong. In the novella itself, this mouse manifests itself as mice as well as rabbits, Lennie's puppy, Candy, and, ultimately, Lennie. Lennie, unaware of his strength, inadvertently kills whatever small, weak creature is unlucky enough to cross his path. Likewise, Lennie is weak-minded, and those who have a stronger mind take advantage of him. For example, Curley, smaller and weaker in size, has a lot more mental power than the gentle giant, so he is able to tease and bully him. Similarly, Candy is weak in that he cannot stand up to Carlson when the latter wants to kill Candy's dog. "He looked for help from face to face," but ultimately, the dog is shot.

Finally, Candy's dog itself is a symbol. The decrepit, useless animal represents society's view and treatment of old, disabled people. Candy acknowledges that he, like his dog, is old and disabled, having only one hand. He asks George, "You seen what they done to my dog tonight? They says he was no good to himself nor nobody else" (60).