When Lennie is hiding out at the riverbank campsite, he imagines that he is visited by his Aunt Clara, described as a little fat old woman wearing thick glasses and a huge gingham apron with pockets. She berates him for getting himself and George into trouble when George had always been so good to him.
Lennie moaned with grief. "I know, Aunt Clara, ma'am. I'll go right off in the hills an' I'll fin' a cave an' I'll live there so I won't be no more trouble to George."
"You jus' say that," she said sharply. "You're always sayin' that, an' you know sonofabitching well you ain't never gonna do it. You'll jus' stick around an' stew the b'Jesus outta George all the time."
Although it is his Aunt Clara who seems to be talking to him, she is saying things he has heard from George, and she is using some language his Aunt Clara would probably never have thought of using. The old woman vanishes, and Lennie is then visited by a gigantic rabbit with whom he carries on another conversation.
As explained in the Introduction in the enotes Study Guide for Of Mice and Men (see reference link below), Steinbeck planned to turn his book into a play, and the play was produced on Broadway in New York in 1937, the same year the book was published. It is obvious that Steinbeck was making the adaptation simple by using as much dialogue and as little prose exposition as possible.
Since Lennie is alone at the campsite in the last chapter of the novella, he has no one to talk to but himself. He does talk to himself at first--but Steinbeck did not want to use too much of that because it would sound unrealistic and might sound like old-fashioned soliloquies in such plays as Hamlet. At the same time, something needed to be put into words, because the author couldn't just have Lennie sitting there in silence waiting for George. It would be a dead spot in the play. The audience could get no information about what was going on in Lennie's troubled mind just from looking at him sitting hunched over and saying nothing.
So Steinbeck came up with the solution of having Lennie conversing with an imaginary Aunt Clara and an imaginary "gigantic rabbit." This would be plausible given Lennie's feeble-minded condition, and the two hallucinations would be easy enough to represent on the stage with a live actress and an enormous stuffed rabbit. Both Aunt Clara's and the rabbit's dialogue would be spoken by Lennie himself, as Steinbeck specifies in the novella.
And when she [Aunt Clara] spoke, it was in Lennie's voice.
Aunt Clara was gone, and from out of Lennie's head there came a gigantic rabbit. It sat on its haunches in front of him, and it waggled its ears and crinkled its nose at him. And in spoke in Lennie's voice too.
The audience would understand that both the old woman and the big rabbit were Lennie's hallucinations, mainly because they would be hearing Lennie's familiar voice speaking his own thoughts and those of his two imaginary visitors.