A Zanbatō (斬馬刀, Horse-Slaying Sword) is an ancient Japanese longsword devised even before the Sengoku Jidai. With its long, spear-like handle and even longer double-edged blade, a zanbatō was designed to take down a samurai on horseback - along with his horse - in a single swing, but due to its unwieldy size and a weight decidedly greater than any other katana, it is said that no one has even been able to brandish this unusual blade to its full capacity. However, this unique weapon is the trademark armament of Sagara Sanosuke.
Compared to historical zanbatō, the one Sanosuke wields is unrealistically oversized, with a full length nearly twice his height and a blade several times wider than normal. He claims that his "partner" is an antique from the Ōnin War, making it over three hundred years old, and its age is apparent from the faded weapon's multiple fractures. Due to the blade's complete lack of an edge, it can no longer cut and can only be used to smash or crush opponents and objects, however, it is shown cutting Hiruma Gohei's cheek when thrown. Sano's great strength allows him to wield his zanbatō with relative ease but its size limits it to simple vertical and lateral attacks, making it easy to read.
At the end of his first duel against Himura Kenshin, Sanosuke's zanbatō is cut in two near the hilt and retired. However, it reappears in Act 185 when the Kamiya dojo comes under attack by Kujiranami Hyōgo of the Six Comrades. To combat Kujiranami's cannon, Sano has refastened the pieces of the zanbatō's blade with iron bolts, but the blade breaks in two again when Sano successfully uses it to deflect a cannon shell. Sanosuke uses the stunted zanbatō one final time against Kujiranami's comrade Inui Banjin, shattering the man's Invulnerable Gauntlets; however, the powerful strike also shatters the remains of the sword and Sanosuke is forced to say goodbye to his long-time battle partner.
In the kanzenban version of the manga released in July 2006, Watsuki included a possible redesign for Sanosuke, in which he wielded a more historically accurate - and, therefore, smaller - zanbatō. A more accurate and wieldy has also been featured in production stills for the Rurouni Kenshin (Live-Action Film).
The sword itself is likely a reference to the ginormous sword Dragon-Slayer, wielded by Guts from the Berserk series/franchise by Kentaro Miura. While the zanbatō is not nearly as long, large, thick, or heavy as the Dragonslayer, the two have notable similarities, most notably how they are both large and are notably deemed "unwieldy". The biggest difference they hold would be in the fact that Zanbatō can be wielded by a more average man and can be damaged. Dragon-Slayer on the other hand is incredibly reinforced both physically and astrally to where it cannot be broken by any normal ability, or technique, and the fact it can only truly be wielded by a man with the strength of ten men, and the strength of many demons from bathing the sword in the blood of a thousand demons and it's owner in the blood of hundreds of men. Another notable thing is the appearance of the Zanbatō, where it looks much less like it's historical counterpart, and more like Gut's sword, most notably in the fact it's blade is wide and double edged, making it look more European rather than Asian. Also, the fact the Zanbatō can only be swung from the sides or top, unlike Dragon-Slayer in its owner's hands where it can be swung from any direction so long as it's wielder is fit to do so.