Katana

The Katana is a long Japanese sword used by the Imperial Japanese Army in Call of Duty: World at War, and usable in Call of Duty Online. It also makes a brief appearance in Call of Duty: Black Ops, Call of Duty: Black Ops II and Call of Duty: Black Ops III.

Call of Duty: World at War
The katana is unusable by the player and only seen being wielded by some Japanese soldiers during certain parts of the campaign mode. In the first mission, "Semper Fi", a Japanese soldier playing dead stabs Rooker with a katana. Another slashes Miller across the shoulder with a katana as he tries to escape with the other Marines. Also, in the last part of "Little Resistance", Sgt. Tom Sullivan gets killed by a Japanese Banzai Charger wielding a katana. Katanas are usually replaced with the Bayonet by the Japanese soldiers, however they can be seen on some of the soldiers' uniforms even if they are not used.

Examination of the game files reveals the menu and pickup icons for the katana, hinting it was originally going to appear as a usable weapon at some stage of development, at least until it got scrapped.

Zombies
Takeo Masaki has an unusable Katana on his character model in Call of Duty: World at War, Call of Duty: Black Ops, Call of Duty: Black Ops II and Call of Duty: Black Ops III. He is seen slicing zombies with it in the cutscenes of Origins, The Giant and in Der Eisendrache

Call of Duty Online
The Katana is available in the multiplayer mode of Call of Duty Online.

Trivia

 * Tank Dempsey occasionally makes remarks about how great it'd be to use Takeo's Katana, but then points out it must just be for decoration, much to his disappointment.