I have an old friend.
A real friend indeed.
We have fought to the end.
On and off a raging steed.
What I am and what I am not.
I am protective for all can tell.
And as to why I worry a lot.
I fear my friend is not well.
For I have heard he likes to draw.
And drawing is the only thing he'll do.
But his drawings distresses all.
He is cold, fierce and cruel.
You are most likely
a shield. You protect, and are used by a knight who fights with sword and shield, both on horseback and off.
Your friend is most likely
the knight. You have fought to the end with this knight, who now is not doing well (perhaps having lost a battle, or at least gotten injured in one).
Your friend is most likely drawing
a sword, in the to pull from a sheath, as a sword sense. Pulling a sword out is likely to distress others, and swords are "cold" metal, are "fierce" weapons, and can be thought to be "cruel".
Why I am slightly uncertain:
It's unclear why a shield would be worried about a knight drawing steel, or would consider the knight unwell because of this. It's also unclear why the knight is described as cold, fierce, and cruel, as that sounds like a more fitting description of the sword the knight seems to be drawing.
Is your friend
The friend is some kind of knight or something (fights on and off a steed). The speaker is a shield or armor (protective). The cold, fierce and cruel knight is distressing people by "drawing". In this case, since the knight is cruel, people would rather he be dead, so he is distressing people by continuing to draw breath. The "fought to the end" and "friend is not well" parts may refer to the knight drawing their last breath. Also fits the wordplay tag because of the less common use of drawing.
I think you are
and your friend is
a medical blood lancet, possibly in a pack, or as part of a glucose meter
We have fought to the end / On and off a raging steed.
You use your medical equipment to administer healthcare to patients both outside and inside the back of an ambulance, and sometimes with unsuccessful results ("to the end" of the patient's life)
What I am and what I am not / I am protective for all can tell.
Healthcare workers and equipment heal and protect
And as to why I worry a lot / I fear my friend is not well.
You might be worrying that it's defective equipment
For I have heard he likes to draw / And drawing is the only thing he'll do.
Literally the only purpose of a medical lancet is to draw blood
But his drawings distresses all / He is cold, fierce and cruel.
Most people don't enjoy being prickled with needle-like objects made of cold metal with pointy aggressive shapes