If the Dread Knight visits the Necropolis can he take a Warhorse from the stable deck?

I think Yes.

Necropolis states: "When you visit the Necropolis, it is treated as if you were visiting the Graveyard"

Dread Knight card states: "....if you visit the Graveyard"

The wording is identical - visit the Graveyard, so I don't see why you couldn't.

If anyone has another thought on this please chime in!

Perfectly correct answer!