I thought the same thing, but Yaddle was thought up from a piece of concept art of Yoda when making Ep. I. In the original trilogy Yoda was truly the only one of his kind, not just the last. As much retconing that Lucas has done, it wouldn't surprise me if he completely forgot until he had about four more Yoda's running around and just decided they would be a nameless race.

Edit: Hmm, now that I think about it that makes a lot more sense. I suppose I figured "the only one of his kind" meant that it had been a long ass time since the rest of them died out.