[IXRollOutIX]

Simply, it could just be the DNA of one of the Extinct generals. Perhaps one of the scientists in the Paleo development team wishes to study the DNA of their enemies for the sake of SCIENCE! Ray won't do it for him knowing it could be dangerous, so the scientist either consults one of the secondary Riders to do it for him or makes a nefarious deal with the Exinct somehow.

The thing is, by the time the final form is even in question, one or more secondary riders might be in play here. If the final form is really integral to the story (which I think it should be) it might be beneficial to decide who all the characters involved in the plot at that point are before fleshing out all the specifics.

I think we're all in agreement that the final form should be something close to the extinct themselves in some way. So, we should design the generals, decide what the form does and looks like, then come up with the "hows" and "whys" later when we discuss the actual story events in further detail.