Plot twist; the shadow isn't Ridley.
The shadow is the Rhedogian.
-Similar tail tip as Ridley (more importantly, as the bit we see in the Smash Direct)
-Can fly, just like Ridley
-Is encountered in the Pyrosphere (though Sector 3, not the Geothermal Power Plant; despite this, Smash isn't necessarily canon
and the corridor in the big hole looks
kind of like Sector 3 anyway...)
-Has to be fought
four fricking times before it's dead. And then two more show up in the post-credits game. So it's an "enemy of Samus' past"
and a "trauma"