It is the vital plane, probably the vital physical. It is mostly there that the beings of the vital world appear with animal heads or features. A human figure with a dog's face means a vary coarse and material sexual energy. Of course, all such energies can be transformed and cease to be sexual — turned into material strength of some kind, just as the seminal force can be turned by brahmacharya into ojas.
This kind of sexual attack through sleep does not depend very much on food or anything else that is outward. It is a mechanical habit in the subconscient; when the sexual impulse is rejected or barred out in the waking thoughts and feelings, it comes in this form in sleep, for then there is only the subconscient at work and there is no conscious control. It is a sign of sexual desire suppressed in the waking mind and vital, but not eliminated in the stuff of the physical nature.
To eliminate it one must first be careful to harbour no sexual imagination or feeling in the waking state, next, to put a strong will on the body and especially on the sexual centre that there should be nothing of the kind in sleep. This may not succeed at once, but if persevered in for a long time, it usually has a result; the subconscient begins to obey.
A breath of disillusion and decadence
Corrupting watched for Life's maturity
And made to rot the full grain of the soul:
Progress became a purveyor of Death.
A world that clung to the law of a slain Light
Cherished the putrid corpses of dead truths,
Hailed twisted forms as things free, new and true,
Beauty from ugliness and evil drank
Feeling themselves guests at a banquet of the gods
And tasted corruption like a high-spiced food.