Well, there is the theory that the blobs are in fact Mudflubs, and we did technically see the leftovers of Sea Rexes in Munch's Oddysee. But like you say, they'd still be new cast members.
But then so would a Mudokon we haven't seen before. OWI continually call species 'characters', which is cute, but does lead to ambiguity in their statements.

I'm not complaining, though.