Final Theta in Spanish

[Anthony Appleyard]

 "David L. White" wrote, of Spanish words in -z:

 > Why are these words so rare, evidently?  Should not all Latin
 > 3rd declension nouns in /k/ show up with final theta in Spanish,
 > if "lux" and "pax" do?  I cannot at the moment think of any more,
 > but there must have been more than 2 ...

  Why does Latin {rex} = "king" show in Spanish as {re} and not **{rez}?

[ Moderator's note:
  The stem of Latin _rex_ is _reg-_, as seen in the accusative _regem_.
  --rma ]