How to determine if image is something or not?
The best approach, the one that seems to have better results in research and industry is to use a Neural Network, a CNN (convolutional network to be more precise, although a linear forward layer would also work).  
Nothing has changed from cats vs dogs to mouse vs not mouse. Still a classification problem.Since you are using an SVM I assume you don't have a lot of data, maybe try using transfer learning with resnet models ? It might yield good results. 
If it's still not enough, you can still use SVM for classification 1-mouse 0- not a mouse.!
Hope it helped !
Best!
I have a trained model that determines if an image contains either cat or dog. I'm using SVM to classify. I want a new model that determines if an image is a mouse or not. It's different from the first model that classifies into 2 classes. This new model will return TRUE or FALSE only.
I don't think I can use a classifier model since I have only 1 class; the mouse. I don't intend to use the first model or anything related to it in the 2nd model.
What is the best way to approach this?