You don't have to tell me it's a vanishing breed.
A play on the term "vanishing breed". "A vanishing breed" can figuratively refer to any category of something that is decreasing, e.g., "honest politicians are a vanishing breed". But here, "vanishing breed" is literal: the type of horse is "vanishing," i.e., it's literally invisible, and "breed" refers to a literal breed of horse; so both words take on a new more literal meaning.