SQL Sum Aggregation

Perhaps you already have a list of categories in the form of a table or defined in some other way.
Can be used explicitly

SELECT c.category,sum(legs) as legs 
FROM test t
INNER JOIN ( 
    VALUES 
           ('chicken','bird')
          ,('crow','bird')
          ,('dog','mammal')
          ,('cat','mammal')
   )c(animal,category) ON c.animal=t.animal
GROUP BY c.category
;

There
IN(...) -> "INNER JOIN"
CASE ... END -> c.category

Let's say you have an "animalcategory" table or query

create table animalcategory (animal varchar(15),category varchar(15));
-- and possible
create index ixanimalcategoryanimal on animalcategory(animal);
create index ixanimalanimal on animal(animal);

Then

SELECT c.category,sum(legs) as legs 
FROM animal t
INNER JOIN animal_category c ON c.animal=t.animal
WHERE category IN('bird','mammal')
GROUP BY c.category
;

This approach will be much more productive on big data than using the CASE...END. It will be possible to use indexes.

Fiddle

Oracle 23.9 and newer supports GROUP BY ALL:

SELECT
    CASE
        WHEN animal IN ('chicken', 'crow') THEN 'bird'
        WHEN animal IN ('dog', 'cat') THEN 'mammal'
        ELSE animal
    END AS animal,
    SUM(legs) AS legs
FROM t1
WHERE
    animal IN ('chicken', 'crow', 'dog', 'cat')
GROUP BY ALL;