"Hey Bulldog" (The Beatles)

"It's a good-sounding record that means nothing."
This is how John Lennon described his song "Hey Bulldog" in a 1980 interview with journalist David Sheff.
The song began as a rough sketch called "You Can Talk to Me."  Lennon hadn't planned on doing much with the tune, until the band was set to shoot a promotional video in the recording studio for Paul McCartney's new single, "Lady Madonna."  Rather than miming to the "Lady Madonna" track, McCartney convinced his bandmates that they should work on a new song and have the videographers film that instead.  So the boys worked on "You Can Talk to Me" while the cameras ran.
When McCartney spontaneously started barking after the last verse, Lennon ad-libbed, changing the next line of the song from Hey bullfrog! to Hey bulldog!, which became the new title of the song.
The song never was released as a single, not even as a B-side.  Ultimately, it wound up as a track on the Yellow Submarine soundtrack album (1969) but was cut from the movie to help reduce the film's running time.
For as much of a throwaway as Lennon considered the song, it is a perfect example of late-period Beatles: there are still tinges of psychedelia leftover from the Summer of Love (mainly in the abstract lyrical approach), but the music is straight-ahead rock with a bass- and drum-heavy R&B feel.  The opening riff, played on piano and guitar by Lennon and George Harrison, respectively, is like a close cousin to Barrett Strong's 1959 hit, "Money (That's What I Want)," with a little bit of James Bond swagger thrown in.  Ringo Starr also does a solid job as always, never getting too showy or adding needless fills; it's just steady groove from moment one.
And then there's McCartney's bass.  
It is one of the best basslines ever recorded.  The supple tone, the syncopation, the inventiveness of the little octave jumps, runs, and slides.  It's funky, melodic, rocking, confident.  It also perfectly fits the buoyant mood of the song and elevates it to another level without showboating.
Most artists only dream of having "throwaway" material as great as this.