In answer to your question - no, you don't need to tag speculation and it's best not to because it could lead to your ideas being taken as fact (also, those of us who avoid "spoilers" are unlikely to open the tag or reply to your posts cos - stupidly - the tagged content is shown in full during editing if the post is quoted)

As to who or what was under the blanket - the key point is that the Doctor was sitting in the chair in that room and we don't know when he entered the room. If he was there before the "visitor", he'd know if it walked in and crawled under the blanket - otherwise, he came into the room whilst that thing was already in the bed and didn't care - none of the "ooo - I'm scared" routine he pulled later.

Could it be the Doctor or Clara? Anything's possible if explained away in the script but there's a problem in that it requires one of them to have been in that room twice at the same time - and that's usually a no-no in Doctor Who.

The most obvious answer is that it was another kid and they either didn't notice the Doctor sitting in the chair or - more in keeping with the Doctor pulling tricks to scare/con people in that episode - the Doctor actually TOLD another kid to do it. That would explain why the kid didn't run away as soon as it saw an adult in the room and just stayed there until the Doctor effectively said "you can go now".

It was deliberately ambiguous - that was the really beauty of he whole story - but I'd have to say that we know that "thing" was heavy enough to make the bed sag when Clara was underneath - and it would be very unusual for a bogeyman to be shown as heavy in that way.

I think that's one of those thing's where it's a problem when they want it to be for a story like fathers day or for explaining why the doctor cant just go back to an earlier time to sort out a problem, but is also something which they would conveniently forget about or brush away with a one word explanation if they needed it to be okay for a particular story.

After all, let's not forget every multi doctor story is the doctor crossing his time stream and interacting with an earlier version of himself and we don't see reapers or anything of the sort on those occasions.

That's the answer then - it was the 11th Doctor paying another surprise visit.

Perhaps he's in every episode and you have to spot him - hence the reference to "Where's Wally".

Yeah, a nice little in - episode easter egg for the fans

Even though I suggested the idea, that's not because I want it to happen, and I'd even be rather disappointed if that does turn out to be the case. It's more that I found myself thinking "yes, that's just the direction they (he) would take it".

I'm in two minds. On the one hand, it's creepier if it's never explained, if it might have been another kid all along, if it was just the hull cooling down. But... is "a little boy who's scared of the dark" really the Doctor? Well, he might have been once, and it certainly looks as though he's channelled those fears into something positive for the main part, but he did seem more badly rattled by the situation than we've come to expect. And another thought - after visiting the barn, the Doctor seemed awfully willing to just let things lie, which seemed at odds with what had gone before. Unless he had seen something at the end of the universe.

Hmm, it really was an episode with more questions than answers!

Also, there's an obvious question as to how it became so universal - psychically transmitted to the mind of every living being throughout time and space in some way, I suppose. I don't necessarily want an answer to that question, but I would at least like to know that there is an answer, and that it's been well thought out!