I've been learning adjective conjugations and I've understood that to conjugate an adjective into its negative form you drop the last syllable (～い syllable) and add くない. However, I've run into the word すくない which has left me puzzled. It seems to be the negative conjugation of a word but I don't see which one.
As answered, 少ない is not a negative form of an adjective, but rather already the dictionary form.
It is only coincidentally pronounced the same way as the negative form of other い-adjectives.
It might be interesting to know that the dictionary entry of すくない also shows alternative kanji (including archaic versions), where the only kana visible after the kanji is い. So it appears the stem of the adjective stops at すくな.
Further, some dictionaries even list it as 少い, albeit as an alternative form of 少ない.
So it appears the word originally, or principally, came in the form 少い, but because of the ease of misuse/confusion, adopted the more verbose version 少ない as the dictionary form.
少ない - 原則通りであれば「少い」となるが、この送りがなで打消表現を書くと「少くない」となって、これが却って「すくない」とも読まれる可能性がある。
Some other adjectives that behave like this, but do not appear to adopt the same usage are: