I'm guessing you've seen Destroy All Monsters and that's been your exposure to Manda?

Manda is awesome...too bad he was only in a few scenes like you said. You can see the movie, Atragon, for Manda's original appearence...though it's just as brief as in Destroy All Monsters! Sadly. You'll also notice some differences in the prop used for Manda...originally it had a crest of horns on it's head like and asian dragon in Atragon. Anyways...the movie your looking for is Atragon, a good Sci Fi film in it's own right, but not much of a monster movie.