Counterpart Comparison: With Link. Predating The Legend of Zelda by a year, Taran wears clothes and carries a sword strikingly similar to those of Link in his very early years. Not to mention looting dungeons, working well with animals, wielding a magic sword, having to stop a very old Big Bad from getting an artifact of great power, and breaking lots of pots.
Peter Jackson seems to have taken inspiration from Gurgi...
Ensemble Darkhorse: The Horned King. Even if the movie failed at the box office and is one of the lesser known Disney feature films, The Horned King is a cult favorite within the Disney fandom and many times is compared with the likes of Maleficent and Chernabog. Probably the only reason why this movie is his only appearance within the Disney universe is because he's Nightmare Fuel incarnate and so would traumatize younger viewers in further appearances. Fans have even already demanded Disney to let The Horned King to make a main appearance in future Kingdom Hearts games. Being played by John Hurt probably has quite a bit to do with it. He had a cameo in House of Mouse, for what that's worth.
Older Than They Think: Everyone seems to think The Little Mermaid's Ariel started the trend of assertive Disney heroines — see the Rebellious Princess entry. But it may have started with Eilonwy here. Glen Keane, who later made Ariel, even designed her.
Quality by Popular Vote: it could be argued that the film has suffered from this. Because it was unpopular in the theaters, it's been severely underrated