One-class classification in Keras using Autoencoders?
If you have images of cats only, you could create boundary boxes (BB) of your images. Some BB will have cats an others won't. You will label those BB with cats inside as 1 as the others as 0.
This way you can set up a dataset with a binary class. It will be much easier if you already have boundary boxes for the cats in each image since this way it will be possible to generate the dataset automatically (generate random BB and label them as 1 if the intersection with the cat is big enough and 0 otherwise).
There is some software that helps to create boundary boxes.