Lennie sees two different items, well one actually transforms into the other. He sees his Aunt Clara and he sees a human-sized bunny.
I think he sees the Aunt because she represents morality to Lennie. He knows he has done something wrong and the ethos in him is triggered. The rabbit to me represents the passion of his or that which makes him happy: soft things, and the hope of livin' off the fatta the lan' one day with George.
Without George to direct his thoughts after this severe event, I think his brain's strongest thought processes took over.