Little Norse Prince Valiant
Also known as The Great Adventure of Horus, Prince of the Sun because international roll-outs were a bit less consistent in the 60s. While Miyazaki made his first feature with Lupin, this is actually the directorial debut of another Ghibli founder, Isao Takahata. Depending on which dub you find, it can play like a harrowing tale of revenge and survival… as produced by Ruby-Spears (who you may know as the producers of the Thundarr the Barbarian, Heathcliff, and Mister T cartoons).
Little Norse Prince is a very particular concoction of adult themes and childlike earnestness, following a sort-of junior Siegfried, “Hols,” who journeys across a Nordic fantasy realm that Beowulf might cohabitate. The fact that Hols’ sidekick is a cute bear makes this seem like a carefree kids adventure at first; until it’s made clear that the young hero is seeking vengeance on a wicked warlock who razed his village. Yikes. The gauntlet Hols must endure involves betrayal at the hands of a love interest, jealousy from the very townsfolk he saves, and some really gnarly battles with legions of wolves.