Happy Howloween!

In fevered pitch
I groan and twitch
As pain bestrides
My body's width

With maw outstretched
I yowl and wretch
A grizzly sight
A sickening stench

Then burning rage,
Unleashed!
Uncaged!
My mind: deranged!
The final stage;

A beastly being
A moonlit scene...

Say, what am I,
This Halloween?