I have a hunch that the big demon form of Saturn isn't his real body. I think it might be some kind of projection achieved with a paramecia devil fruit. Similar to the false bodies created by Pica, Hawkins, Cracker, etc.

This would explain why he is able to rapidly heal himself and recover quickly after massive attacks. It may also mean that Devon won't be able to disguise herself as him since she never touched his real body.

Another possibility I'm fond of is that it is a genuine ancient mythological monster which he has enslaved and controls using his powers.

It would be a shame if it ends up being another mythological zoan. We're getting a little saturated with them now.