Aerilyn said not to discuss type changes in that thread, but I just wanted to point out, Gyarados supposed to be Water/Dragon in Gen 1 was a rumor. There wasn't any actual interview where that was said. I think that rumor started 3rd Gen when word of the remakes were coming out. Since Kingdra was Water/Dragon in Gen 2, the rumor about Gyarados being retyped started then.
That's where I first remember hearing the rumor at least. But ya, there is no official statement that says Gyarados was originally to be Water/Dragon.
Its part flying because of the mythos that surrounded its creation since that dragon could fly.