What is this enemy called in Dark Souls? Looks like a huge dark hell spawn knight

Solution 1:

I'm really not so sure what you're talking about either...are you referring to the Dark Knights?

If so, these guys are pretty powerful at the point of the game you're at - they're placed in locations that you can easily avoid, so for the moment you can run past them and come back to fight once you're stronger.

They also don't respawn so you don't need to worry about having to fight them again after you take them down once.

Solution 2:

The creature that fits your description is a Black Knight and the place you refer to is the watchtower that stands by bridge where the Red Drake lies, in between Undead Burg and Undead Parish. I suppose you are around level 15 and yes, this area is meant for your level, but Black Knights are slightly powerful than normal foes, that's why engaging them is optional.

Go to Undead Parish first and come back later when you have leveled up a bit. You will have no trouble defeating the Black Knight then.

Black Knights are considered mini-bosses and they do not respawn after they are dead (Except for an area where they do respawn, late in the game).