Description says, "Sprayed on the ground, it creates wall of fire that does 50 points of fire damage per second."

I saw a pit dog/wolf in a cage and decided to try my new dragon priest staff. The whole cage inside was on fire for a LONG time but the pit dog/wolf only lost small amount of health at the beginning....then it was just standing there in the flames without losing any health.

I was excepting that anyone standing in the burning area would constantly take damage, you know the "50 points of fire damage per second"

Might be a glitch with the cage/dog. Try it on another enemy and see what happens.

It was probably the cage that caused this, try it on open ground

Well, the initial damage is on direct hit...after that, it's only a lower fire damage...but still...