Hey, hey, hey! Before he launched Fat Albert & The Cosby Kids, Bill Cosby began his association with Filmation with a primetime special, an adaptation of Aesop's Fables.
Cosby has the title role in what appears to be a pilot for another series, this one a collaboration between Filmation and Lorimar. Jerelyn Fields (Curiosity Shop) co-stars, and would return to Filmation 10 years later to work on The Kid Super Power Hour with Shazam! for NBC.
I should note that Cosby's first project for CBS may have been the TV-movie, "To All My Friends on Shore". I'll have to check that.
Anyway, Filmation regulars Jane Webb, John Erwin, Dallas McKennon, & Larry Storch provide the voices for most of the animated characters, and I'd not be surprised that co-producer Lou Scheimer may have contributed as well, uncredited. Speaking of uncredited, that brings us to the idea that the talking tree's voice may actually have been that of Ted Cassidy, who'd later return to Filmation for a guest gig on Tarzan, Lord of the Jungle. Cassidy is not listed on any source guides.
Sounds like Larry Storch is the hare. John Erwin is definitely the fox, as there's no mistaking his voice. The Tortoise & The Hare had been adapted rather loosely for shorter pieces by Warner Bros. (2 Bugs Bunny shorts) and Jay Ward (Aesop & Son), but this may have been one of the more definitive adaptations.