Any place name followed by {-ngan} is a demonym. The only exception is that a {tlhIngan} apparently is not a native of any place named *tlhI* or *tlhIn* or *tlhIng*, and we have not seen any references to a {Qo’noSngan}. One assumes that this must be something like the way tera’nganpu' don’t commonly refer to the Sun as a star. Similarly, we say that those from Venus are Venusians and those from Mars are Martians, but we say we are from Earth, yet we refer to ourselves as “terrestrials” and not *Earthians*.  When we talk about home, exceptions are made. Likely, it’s similar for Klingons.

While TKD did give some examples of both planet names and demonyms of those from those planets, it hasn’t been a persistent trend.