Terrier is a label for a group of dogs rather than one breed. They were developed over several centuries in Britain to hunt rats, foxes and other vermin, often by following prey underground, which is where the name, from the Latin for earth, comes from. Listings use it for everything from tiny Yorkshire-type dogs to sturdy Jack Russell, Rat and Cairn mixes.
Most adoptable Terriers are small to medium, roughly 10 to 35 pounds, with wiry or smooth coats and a bold, loyal streak. In O'Fallon, that likely means a dog that announces every visitor and tests the fence line, so a secure yard and steady training matter. Size, coat and energy vary with ancestry, so meeting the dog says more than the label.