There are no prototypes. It's speculation based on the fact that every other character has a legends figure.

And Ironhide has a fairly simple transformation.

EDIT: I'm all kinds of wrong. See below.

 

Yes there is. Ironhide was shown off at the Hasbro tour at Botcon alongside a bunch of other prototypes. I'll find the pics.

http://www.tfw2005.com/gallery/data/1806/medium/40597468531ba7bc98.jpg

Next to the Evac test shot. There's also a robot mode picture, but not in this gallery as far as I can tell.

http://www.tfw2005.com/gallery/data/1806/medium/Hasbro_tour-Botcon-0053.jpg

A robot mode shot, but not the clear, centered one I'd seen elsewhere.

http://www.tfw2005.com/gallery/data/1806/medium/PICT5143.JPG

Still not it either, but very close.

 

http://www.tfw2005.com/boards/showthread.php?p=1584578#post1584578

Are those the ones you're looking for?  

 

Yeah, those were the ones! So strange that nobody else ever took better pics. Guess others were mesmerized by Evac or something.