Jetta has two primary associations: the Volkswagen Jetta (in continuous American production since 1980) and the name's own sound quality: jet-black, fast, feminine ending. Both associations produce the same image: sleek, dark-coated, and moving with purpose. For a black female dog, it's practically self-explanatory.
The Car Name Tradition
Vehicle names on pets are usually chosen for the sound or the specific quality the vehicle represents. Jetta sits in the speed-and-sleekness category rather than the rugged-utility camp. Owners tend to favor lean, athletic dogs: Doberman Pinschers, Whippets, Black Labs.
The Coat-Color Connection
Jet black is an ancient color description, and Jetta carries that color meaning subliminally. On a black dog it works as both name and description simultaneously. On a dog of any other coat color, some of that logic dissolves.
The Counter-Reading: The Car Will Date It
Any name tied to a specific car model risks aging if the model is discontinued or becomes associated with a particular decade. VW Jettas have been around long enough that the reference is stable, but it's worth considering whether you want your dog's name permanently associated with German automotive engineering.