Tancho isn't a separate variety so much as a marking that can appear across several varieties, and it might be the single most recognizable pattern in all of koi keeping. A true Tancho has an otherwise white body (or white with black or metallic accents, depending on the base variety) with exactly one round, solid red marking centered on the head, and nowhere else on the body. The name comes from the Tancho crane, Japan's national bird, whose red-crowned head the marking is meant to resemble. It's the same image used on the Japan Airlines logo, and the same red-on-white symbolism found on the Japanese flag.
Because a Tancho is defined by where the red appears rather than what variety it belongs to, you'll see Tancho Kohaku, Tancho Sanke, and Tancho Showa, each judged first as a member of its base variety and then held to the added standard of that single, isolated head spot. A Tancho Showa, for instance, still needs the black sumi and white base expected of any Showa, on top of getting the red marking right.
Getting a genuine Tancho is difficult, and that difficulty is exactly why the variety carries such prestige. The spot has to be round, not oval or blotchy, it has to sit centered between the eyes rather than off to one side, and it can't bleed down toward the eyes, mouth, or gill plates. Breeders can select for koi with head-only red markings, but locking in a perfectly round, perfectly placed spot generation after generation is closer to luck than to control, which is part of why a high-quality Tancho commands a serious premium over an equally well-patterned koi without one.
For collectors, a great Tancho is as much a symbol as it is a fish. It's the marking most non-koi-people recognize on sight, and it's often the one variety a hobbyist will pay far more for than the underlying pattern would otherwise justify, simply because of what that single red spot represents.