Obscurity factor: 9
Not to be confused with the 2001 book of the same name by Lawrence Anholt, this 1968 short dates back to the reorganization of the Warner Brothers animation studio. It's a one-off, featuring a boy, his rare blue-tailed monkey and a hunter who wants the monkey as a trophy. It's the usual cartoon fare, with suspended boulders, hollow logs and angry elephants, but has the distinction of being voiced by Mel Blanc.
It's unclear whether this one is available on DVD in one of the WB compilations, but you can watch it in its entirety on YouTube. It's above, uploaded by jgbennie.
Obscurity factor: 9 (orphaned characters, never seen again, largely forgotten, available on YouTube)