Sheep, Wolf, or Sheepdog?

Sometimes I'm like a lowly sheep
Just going here and there,
To find my food and go to sleep
Although, I don't know where!

Sometimes I'm like a wolf in search
Of unsuspecting prey.
My speed and sense of smell won't smirch
My predetermined way.

I'm also like a sheepdog who
Is brave enough to fight
The wolves who often try to do
Some harm to those in sight!