Hmm. They never mentioned anything about there being another of his species in the game.
I guess it's possible that that reproduce asexually. But it doesn't seem likely. I mean, he calls his parents 'mother' and 'father.' (I think...^_^; )
But maybe he just calls them that because the people around him call their parents that? But then, how would he know which one is 'father' and which one is 'mother?' Wouldn't they look the same? (Or at least VERY similar...) I don't know. Squaresoft never explains it. xD

I was thinking maybe somehow someone created another one by cloning RedXIII? (Or something like it.) But then, Hojo is dead, and I don't think there are any other people who would even try to do something like that RedXIII... Unless the little RedXIIIs (his kids) were created through science, and they're clones of RedXIII? But is it possible to clone animals? And if anyone could clone RedXII, then wouldn't Hojo be able to do it? If so, then why didn't he?

I wonder why they don't explain it in the newer games and movies and stuff... Maybe they didn't really think about it when they put the scene in, so there really is no explanation for it. But that's unlikely, since the rest of the game was so well thought out.