They had a bunch of decommissioned animal animatronics laying around after Disneyland closed the show America Sings. Imagineer Tony Baxter proposed taking those animatronics and building a log flume attraction (then called The Zip a Dee River Run) around the music of Song of the South—while the movie was controversial, Zip a Dee Doo Dah has always been a hit among the public. Michael Eisner brought his son, Breck, to work one day and when Breck came across the Splash Mountain model, he loved it. The rest as the say is history.
^^At least, that’s the legend behind the ride, anyway.
@marni1971 can correct the record if I’m wrong.