What could explain a strange sighting like this?

From a distance, in poor light, it can be hard to tell whether a shape is an animal, a person, or a shadow. Quick movement and a brief change in posture make details even less reliable. That does not mean the person imagined the whole thing, just that the description may not be enough to identify what they saw.

The reflective eyes could fit an animal catching light, but they do not settle the question by themselves. With no clear view or other evidence, I would keep the explanation open rather than name a specific creature.

My plain read is that it could have been wildlife moving through the clearing, with darkness making its shape look more human. A startled animal can change posture or pause, and eyeshine can look startlingly bright. That is a possibility, not a confident identification.

The memory deserves to be treated as a real recollection, but not automatically as proof of an unknown being. Seeing something alarming as a teenager, from the edge of vision and at night, leaves a lot of room for uncertainty. I would separate what was noticed, movement, a brief upright shape, reflected-looking eyes, from the interpretation that it was inhuman.