Appearance: Marvel Comics #1 (November 1939).
Golden Age Appearances: Marvel Comics #1, Marvel Mystery Comics #2-27, Human Torch Comics #5.
Modern Appearances: None.
Years Active: 1939-never (see below).
Ka-Zar is David Rand, a...heck, let's be honest, he's a pretty transparent Tarzan clone, is what he is.
John Rand, "young owner of a rich diamond field in the Transvaal," is flying to Cairo from Johannesburg with his wife Constance and young son David when their plane develops motor trouble and is forced down in the "thick, wild TROPICAL FOREST!!" of the Belgian Congo. John and Constance die soon afterwards, from a "raging fever" and a broken heart, leaving David alone to raise himself. When he's a young adult, similar in skills and dress to Tarzan (go figure), he befriends the elephant Trajah and saves the lion Zar, the "lord of the jungle," from quicksand. This puts Zar in Rand's debt, which Zar pays back by tagging along with Rand on various adventures and acting as his leonine sidekick. (Rand later befriends Sha, Zar's mate, thus giving him two lion friends.) Ka-Zar's skills are of the Tarzan variety. In most issues Ka-Zar has no superpowers, but in Marvel Mystery #26 he is given a brew by a witch-doctor which grants him enhanced strength and endurance; the following issue he takes on a German submarine and then is seen never more.
In modern Marvel continuity the Golden Age Ka-Zar never existed. The Golden Age Ka-Zar appeared in Human Torch Comics #5, where the Torch warned him about Namor's tidal waves (thanks to Antoine Verville for pointing this out), but it is current Marvel policy that the only Ka-Zar who exists in continuity is the Silver Age Ka-Zar. The Golden Age Ka-Zar, David Rand, has been retconned away, and thus can't ever exist. Unlike most of the other Timely characters on this site, there's an active Marvel Comics policy against the GA Ka-Zar's existence.