What is the only noun in the English language whose singular and plural forms have no letters in common?

Answer 1

KINE (a plural form of COW) is obsolete.

Since cows is also an acceptable pluralization of cow (as in "He has three cows"), and your question asked for:
"...the only noun in the English language whose singular and plural FORMS have no letters in common"
the question is faulty, and lends one to believe that any plural form may be used for a particular singular noun to fulfill the criterium.

Otherwise there are hundreds, including:
lamb/sheep, hog/swine, me/us, I/we, human/people, etc...

For each of these, the plural DISTINCTLY and SPECIFICALLY refers to the singular (as in, there isn't a non-human singular for people, because a person is human -- there is a non-lamb singular for sheep, but because all lambs are sheep, the plural can distinctly refer to the singular), just like cow/kine.

This is not the case for things like grape/bunch, or cookie/batch because batch and bunch do not DISTINCTLY refer to their singular forms.

...or was this a riddle?

Answer 2

Kine.

Answer 3

Though it is not a noun, you can think of I and We. (I is a pronoun.)