"Small vermin" refers to small animals that are considered pests or nuisance creatures. They are often unwanted and can cause damage or spread diseases. Full definition
Farmers used it to protect their stock and crops from rats and other small vermin. (puppy-training-solutions.com)
In fact, once classified as a working dog, it was originally bred for hunting small vermin and has a terrier's aggressive impulses toward small animals. (pbs.org)
The breed was originally used as a hunting dog for some types of small vermin. (yorkiemag.com)