Ok, this one has been bugging me for ages, and I'm not really sure where to turn.
The above is the card - there used to be an archive with scans of every
card in the series online long ago, but as you probably are aware many of the interesting parts of the internet have since died and gone to hell. But even then, there was never a caption to tell what movie or serial this was from. I know Rosan's Terror Monsters
series included a lot of 2D art and photos from costume contests and whatnot, but I don't think this is in that vein (although I do think the more film-centric Horror Monsters
still included a few ambiguous pieces).
So, what's this weird seahorse from?