Manda, the "thousand meter serpent" as it was described way back in an issue of Japanese Giants (maybe #6, I think) first appeared in the excellent movie Atragon (1963). The monster was kind of shoehorned into the movie, since it really wasn't necessary to have Manda in there, though for us monster loving kids it was always nice to have one show up. Manda really doesn't do much in the movie at all, it's not even a match for the great undersea battleship Atragon- one blast of the freeze ray and Manda is down for the count. Still, it's a great design, huh?
Manda next appeared in 1968's excellent monster opus Destroy all Monsters though with a slightly different look.
I'm reasonably sure this was due to King Ghidorah being in the movie
and having three similar looking heads as the original puppet of Manda
did, so it was given a sleeker, more snakelike appearance. This was the
first way I had ever seen the monster, so it's the one I like the
most. Manda has an excellent seen destroying a monorail in this movie.
Manda also makes a cameo in the wonderful kid's fantasy Godzilla's Revenge
from 1969, but that's just stock footage for Ichiro's dreams. That was
my introduction to the monster, and in a cropped fullscreen view, one
is apt to think Manda is a snake and not a dragon. The last appearance
of the beast (to date) is in Godzilla: Final Wars but I think it was almost all realized in CGI. I'd have to watch the movie again to be sure, which unlike some fans I don't mind doing.