The OED has an extensive etymology that the "x" ending is probably from Scandinavian or Icelandic sources.
My question is, Are there other examples of words taking the "x" ending to make the adjective?
Emmett
I just checked a list of the 300 most commonly used adjectives at: http://www.examples-help.org.uk/parts-of-speech/list-of-adjectives.htm
As you can see, not a single one ends with an "x" and "manx" wasn't even listed, no doubt because it's not that commonly used. I'd never heard it before. Racked my brain and couldn't think of another solid example. Perhaps "six" would count as an adjective in "We saw the six people arrive." Perhaps not. And it's not really taking an "x" ending anyway, which is what you were looking for.
I grepped a word list and found plenty of adjectives that end in "x," (circumflex, complex, simplex, convex) but no others that were formed from a noun by adding the "x." My word list isn't the most complete, though.
Bronx comes close, but I believe it came from "Broncks."
I have found this particular instance described as a unique and irregular formation of a demonym: Isle of Man → Manx.