"Siamese cat" is capitalised, whilst "house cat" is not. You made one of the best points that I saw during the "Genesis Creation myth" debate; that whilst "myth" was technically correct, it was misleading, as the public generally understands myth to mean false. Extremely different scenario; exact same analogy. "Thoroughbred", in most cases, including racehorses, is referring to a purebred horse, and not a specific breed.