"It is hardly a spider!"
Translation:Det är knappast en spindel!
I take it to mean 'I think it's really unlikely that that thing is a spider'.
I tried "Det är knappt en spindel" but it was rejected. Of these two terms (knappast knappt), can only knappast be used to indicate probability?
Yes. knappt is used to mean 'slightly less than' or 'barely', 'not quite', 'just under', 'not more than'.
Both work in a sentence like Det tror jag knappast/knappt 'I hardly believe it' though. (much like you can say I can hardly/barely believe it in English)