Extra Man

Perhaps akin to T. S. Eliot's lines from The Waste Land, "V. What the Thunder Said":

Who is the third who walks always beside you?

When I count, there are only you and I together

But when I look ahead up the white road

There is always another one walking beside you

Gliding wrapt in a brown mantle, hooded

I do not know whether a man or a woman

—But who is that on the other side of you?

In his "Notes," Eliot says:
"The following lines were stimulated by the account of one of the Antarctic expeditions (I forget which, but I think one of Shackleton's): it was related that the party of explorers, at the extremity of their strength, had the constant delusion that there was one more member than could actually be counted."

If there is otherwise an "Arctic myth," this citation is probably irrelevant.