Using table name in column for joining

Hello,

I have a column in a table that contains the name of another table,
and the id in that table.

I would like to use this in a join statement. Is that possible?

example:

create table animal (
id serial,
table_name varchar(8) default 'monkey' CHECK (table_name IN ('monkey', 'elephant')),
table_id integer
);

create table monkey (
id serial,
name varchar(20)
);

create table elephant (
id serial,
name varchar(20)
);

and I want to do:

select animal.name from animal left join animal.table_name on animaltablename.id = animal.table_id;

or something like that.

I know it's probably considered bad SQL, but what else can I do?

Thanks,
Antonio