Open the vial... with unforeseen consequences!

You unscrew the cap of the vial and peer inside. Due to the opaque nature of the vial you cannot see the contents, so you cautiously try to empty some of whatever is inside onto a small plate. A silvery gold powder emerges, and the room fills with a musky scent which makes you sneeze. The powder puffs into the air, getting everywhere, some landing on your face.

As it settles on your skin, you feel a tingle, and then a flush of heat. Your cock stiffens, straining in your pants as it grows almost painfully hard. You retain the presence of mind to put the lid back on the vial and put it down before the sudden and unnatural arousal makes it hard to do anything other than to remove your jeans and pull down your pants to stare in awe at your throbbing erection, the veins running down it pulsing with heat. Unable to help yourself you wrap your fingers around it, and begin stroking, the hot flesh growing larger still, darkening and shifting in shape slightly to something vaguely equine.

Faster and faster you stroke, until you reach an earth-shattering climax, the blunted head of your transformed shaft flaring outwards as thick, pearly ropes of cum spew from it.

You slump in your chair, breathing heavily. With the **** need gone you find yourself able to think clearly once more. In fact you find yourself able to think more clearly than ever, sudden flashes of insight into the nature of the powder coming to mind. You look down at your spent cock and notice that it seems to have returned to a more human shape, though it still looks larger and darker than it did before. Then you look at the mess you made...