You could imagine Selene's ghost bear as having been part of the ghost army he once commanded. It would also fit the warden theme.

But, Kjalnar's might achieve a similar kind of effect, and might be truer to the ghostly army.

Prior Therric or Grundwulf are the more knightly sets I can think of, just off of armor appearance.

When Aragorn took a tumble off a cliff, wasn't it his enemy's wolf mount that went with him? Maybe he wears it now and it looks like the Balorgh helm?

So many possibilities...