riddle 6: answer

Beyond seven mountains
living in a cave,

The seven mountains are the seven classical planets
who are visible from far away. Beyond them it is dark,
like a cave.

are some giants,
and a tiny knave.

Various outer planets are known as gas giants.
And then there is tiny Pluto who made the news lately.

Promoted.

The Outer Planets were promoted to
the same level at the classical planets.

Demoted.

Now one of them is no longer a planet.

With them one is never sure.
They haven’t a meaning.

The source of their meanings is not always easy to follow.

They haven’t a home.

They do not have a domicile. These are occupied by the classical planets.

They are never friendly.

Most of their meanings are unpleasant.

They are best left alone.

Astrology can generally be done without them.