Per Donna Sullivan, President Rat Terrier Club (UKC),
Let's review some of the obvious differences between a Toy Fox Terrier and a Rat Terrier:
TFT Size: Classified by weight - by age of 6 months may not weigh over 7 pounds.
Rat Terrier: Miniature up to 13" - Standard over 13" and not over 18".
TFT body: Appears to be square with height approximate to length. The backline is strong, straight and firm.
Rat Terrier body: Slightly longer than height with 10:9 ratio preferred. The backline is strong and slightly sloping.
TFT Colors: Predominantly white with acceptable colors of white, & black with tan trim; white & black; white & tan with specific markings of black & tan.
Rat Terrier Colors: Hairless - any skin color is acceptable, freckles enlarge with age and skin darkens with exposure to the sun. Coated - White; tri colored (white with patches of black & tan); bi colors (any combination of black, tan, chocolate, red, orange, lemon, or blue with white); the white may be any size and located anywhere on the dog.
TFT Ears: must be erect when alert; are pointed and well up on the sides; set close together; not touching nor set too wide.
Rat Terrier Ears: the V-shaped ears are set on the outside of the skull and may be erect, tipped or button.
TFT Eyes: eye set is prominently forward but not bulging; color is always dark - with as dark as possible preferred.
Rat Terrier Eyes: eyes are obliquely set; round and small - somewhat prominent; rim color matches nose pigmentation; color ranges from dark brown to amber and matches coat color.
TFT Appearance: a small dog that has conformation, characteristics and personality of a Terrier.
Rat Terrier Appearance: a small to medium sized muscular hunting dog that is evaluated as a working Terrier.
While these may appear to be subtle differences to the untrained eye, size, color, body, movement as well as character make each breed unique.