FOR i IN REVERSE 2..13 LOOP  -- i starts at 13, ends at 2
           ................
  END LOOP;
 

13..2 가 아니라 2..13으로 사용하세요....

참고 : http://download.oracle.com/docs/cd/B19306_01/appdev.102/b14261/loop_statement.htm#LNPLS01328

 知音님 감사합니다.

declare
    tmp_i number;
    i_start number:=2;
    i_end number:=13;
begin
    :
    :
for i in i_start..i_end loop
     tmp_i := i_end-i+i_start;  /* tmp_i 를 활용*/
end loop;
   :
   :
end;


혹은

declare
    cursor curList is
    select  13 - rownum + 1 as rno from dual connect by level <= 12;
begin
    for cl in curList loop
           ......       /* cl.rno 를 활용할것 */
    end loop;
end;