Who Am I?

I was once feared by all

Many would tremble at the sight of me

Quickly, I would cross the land

Smaller than most, but fearful I was

Tricks I would play but none would stay

Day after day I am hungry

Up trees and through the grass I go,

To find what will ease my want  

Small food is my desire but blood is my need

I am all around but none see me

For I am smaller than most

If you dare, say my name.