Here's why:
* All horses are technically the same species: *Equus caballus*.
* Breeds are variations within the species: They're developed through selective breeding over time.
So, to answer your question directly, there's no horse that's "not a breed". Every horse you see belongs to some breed, even if it's a mixed breed or a rare, less-recognized breed.
Perhaps you're thinking of something like:
* A horse without a specific breed registration: This might be a mixed breed horse, or one whose lineage is unknown.
* A "wild horse": These are often feral horses, which means they descended from domesticated horses that escaped and are now living in the wild. They may still be of certain breeds, but their breeding has likely become more mixed over time.
Let me know if you had something else in mind!