Secret and dangerous enemy. To dream of finding a rat in the cellar indicates a problem you are unable to see. To kill one symbolizes moral victory.

…Dreaming of rats, denotes that you will be deceived, and injured by your neighbors. Quarrels with your companions is also foreboded. To catch rats, means you will scorn the baseness of others, and worthily outstrip your enemies. To kill one, denotes your victory in any contest. See Mice….