It was Clara's hand on the doctors leg that started his fears, and yet everyone else has also had the same experience as was shown in the beginning and the creatures under the bed looked human from the look of their arms and hands so they could be trapped in time for some reason or it could be the Doctor himself perpetuating the fear but he said that everyone had at some point had the same thing happen too. 

So when we saw the creature sitting or kneeling on the bed, it could look at them and turn its head but it didn't say anything plus when it got off the bed it went over to Clara and Rupert avoiding the doctor, so that may be a future clue. Also, although from the blur it looked like a sontaran, it actually seemed taller and its head was smaller under the blanket..and it moved fast and stole the blanket. Why? Was it naked? Did it like wool/nylon mix fabric? Was it cold? 

If the sontarans are involved, why not come out guns blazing with death or glory as they are built to do, unless this is the only sontaran who doesn't. Maybe it's a cowardly sontaran on the run?

I think that it will be explained later on in the series. Look out for the red blanket people!

Plus, we now have the Toy Soldier sub plot, the fact that Rupert and Clara obviously get together and the end of all things in the universe...there's a lot more to come!