ALTER TABLE tbl MODIFY COLUMN type ENUM('fish','mammal','bird','insect');

Notes

• As with all cases of MODIFY COLUMN, you must include NOT NULL, and any other qualifiers that originally existed, else they will be lost.
• If you add to the end of the list and the list is under 256 items, the ALTER is done by merely changing the schema. That is there will not be a lengthy table copy. (Old versions of MySQL did not have this optimization.)