How to assign variable as column value
Actually DBMS_UTILITY.COMMA_TO_TABLE and DBMS_UTILITY.TABLE_TO_COMMA are a very old procedures. Nowadays people usually prefer this one: 

WITH t AS 
    (SELECT TRIM(REGEXP_SUBSTR('25,27', '[^,]+', 1, LEVEL)) AS n, LEVEL as pos
    FROM dual
    CONNECT BY REGEXP_SUBSTR('25,27', '[^,]+', 1, LEVEL) IS NOT NULL),
a AS 
    (SELECT pos,
        CASE n
            WHEN '25' THEN 'Bird.'
            WHEN '27' THEN 'Elephant.'
        END AS animal
    FROM t)
SELECT LISTAGG(animal, ' ') WITHIN GROUP (ORDER BY pos) AS value
FROM a;