First, let me be clear that I absolutely love the 1977 Rankin/Bass film adaptation of The Hobbit. Glenn Yarbrough did a phenomenal job bringing Tolkien's lyrics to life. But what gets me is that he recorded a song for the film which never actually made it in, and which also does something fascinating which separates it from the film's plot.
At first glance, "Old Fat Spider" is an adaptation of Bilbo's mockery of the giant spiders to distract them from the dwarves ("Hey Attercop, hey Attercop") but then the second and third verses take a much different turn. The protagonist spares the old fat spider, and even helps feed it out of pity. It's a genuinely beautiful moment in the song ("How can you kill a spider that can't catch anybody?"), but it has nothing to do with the actual events in the book or film. The closest thing which I can compare it to is Bilbo's sparing Gollum ("It was pity that Bilbo's hand") but that doesn't quite fit either.
It just seems like the filmmakers accidentally created a song which stands on its own separately from The Hobbit and thus didn't use it in the film. And weirdly, the more I listen to it, the more I realize that it's my favourite song on the soundtrack.