Guess What?

MORE POETRY!
I'm having a lot of fun with this.

Can you guess what is being described in my poem?

Mystery Beast
The beast lies asleep in its cage.
With a rumble and a groan it escapes into the open.
The beast is hungry.
With a loud roar, it travels the greenery for its food.
Its teeth chomp and gnash as it eats.
The monster leaves a path through the green
As it crashes and crunches through.
It traverses the entire yard, not stopping once
In its vicious quest for sustenance.
At last, we herd it back into its cage.
Now, it sleeps.
But soon, our lawnmower will be hungry again.

Did you get it?
--SwimmerGirl