Don't see why it shouldn't be Jabu. OoA establishes Jabu can die and reproduce and do general whale things. Think he or one of his descendants is just as likely as anything else.
The Wind Fish wasn't part of the dream- the Wind Fish was the dreamer itself. And he was seen after waking flying over the sea, so definitely exists in the real world.
And there's precedence for Oceans becoming Deserts in SS's Sand Sea.
So in BotW there are remains of ancient Leviathans (massive whales in this context) dotted around Hyrule. There are three, and they are all slightly different...
One is a huge toothed whale skeleton with eye sockets on the top of its head and a shrine dungeon in its belly.
One has a thick bony...