You appear to be missing the Scout, a basic yellow threat. It's pictured on page 4 of the expansion rulebook, if you want to use some other card as a proxy for it.Thanks, but I do have the Scout (which isn't an expansion threat--it came with the base game). I've since discovered it's the Dimension Spider that I'm missing. I'll probably write to RGG about it.
[Edited to add: I found this by flipping through the rulebook online, so I'm not comparing to my copy of the game-- can't confirm that that's the ONLY card you're missing.]