How did the Doctor recognise the adversary in Heaven Sent?

Question

In Doctor Who episode S9E11, Heaven Sent,

the Doctor has an atypical reaction to the monster when he first sees it. Instead of doing what he always does - walking up to it and introducing himself, he runs away and confesses he's afraid. In other words, he immediately recognises the monster as the being that is going to kill him as soon as it catches up with him. How come? It's not like he met this being ever before, face to face. (Not that the monster has a face.)

We know this, because the Doctor was still alive by the beginning of the episode!

But he knows this is it.

How?

Answer 1

He recognises the Veil as the manifestation of a childhood nightmare.

When he’s facing down the Veil in the bedroom with Clara’s painting, he says:

When I was a very little boy, there was an old lady who died. They covered her in veils, but it was a hot sunny day. The flies came. Nightmares for years. So who’s been stealing my nightmares?

What am I here for? You’ve known about me for a very long time, right?

This is only a minute or so after he first sees the Veil. I think he recognises it immediately, but doesn’t vocalise his thoughts.

I don’t recall more details of this nightmare, but I can’t imagine the dream ended well if the lady caught up to him. Lacking further information, it would be prudent to assume similar consequences for the real version.