This might be persnickety, but "between" implies that there are two things that surround something.
In "A sheep sleeps between this" the word "this" stands for some unspecified object, animal, person, event, time, etc.
Think about this sentence: "The sheep sleeps between this child."
It doesn't make sense. It leaves you wondering: between "this child" and what else?
But these sentences do make sense:
"The sheep sleeps between these children."
"The sheep sleeps between this child and the fireplace."
In the same way, I don't think "A sheep sleeps between this" makes sense, but "A sheep sleeps between these" does.
I think that it might be to do with emphasis. I would guess that in "hiertussen slaapt een schaap", the emphasis is on the location that the sheep is sleeping inbetween, whereas in "een schaap slaapt hiertussen", the emphasis would be that it is specifically a sheep (and not, say, een schildpad!) that is sleeping between them?