Spoilers (OCR needs a better spoiler tag than white text, for dark skin users):
I guess they just wanted it to be a surprise. If he looked anything more Ridley-like than a little fuzzy white chicken, the scientists/Samus might have caught on before it was too late. I will say that from the first encounter, I knew Birdy was going to be some kind of freaky killer. Then it started feasting on that dead creature and I was like yeah...no surprise there. Definitely wasn't expecting it to be Ridley though.
In other news, I just 100%ed the game. The music for when you get the "irreplaceable item" makes me laugh every time, especially the way it ends. Awesome.