Blimey, this kind of thing gets my attention straight away. What sort of size are we talking, roughly compared to the tree line? And was it making any sound at all or completely silent?
I've had a couple of odd sightings out on Dartmoor over the years and the ones that stick with you are usually the silent ones, something about that silence that just feels wrong somehow.
Did you manage to get any photos or video? Even a blurry one can be worth posting up because sometimes people spot things in the background or the light quality can tell you something. What time was it roughly and how long did it hang around before it moved off or disappeared?