A terrier is a dog of any one of many breeds or landraces of terrier type, which are typically small, wiry, very active and fearless dogs. Terrier breeds vary greatly in size from just 1 kg (2 lb) to over 32 kg (70 lb) and are usually categorized by size or function. Included among these breeds are the English Toy Terrier and the Yorkshire Terrier. 2  While small, they retain true terrier character and are not submissive  lap dogs 3  . Bull type Terriers: The Bull and Terrier types were originally combinations of bulldogs and terriers as general mixed breed bull-baiting and pit dogs.