Now the riders of the Second and Third
Horses, brings on war and famine,
and opens the way for the coming of the
rider of the Pale Horse called Death.

It is mentioned that the slaughter is caused
by the "beasts of the earth"; but it's obvious
that ordinary wild animals couldn't do this
kind of destruction.

These beasts are the "beasts" told of in the
thirteenth chapter of Revelation. They will
rule over the earth without mercy killing
everyone who doesn't worship them. But
they only control a fourth of the earth.