You reach down in horror as you realize you penis is now a large rubber horse cock. Not only that, but it's fully erect, and actually made out of rubber. Then it hits you, this is the dildo you bought! With this realization you try to removing, grasping your shaft and pulling hard.
Bad idea.
Instead of removing it, you only sent the biggest wave of pleasure to ever hit your body. Now caught up in the moment you start pumping the equestrian shaft harder and harder. This feels better than anything you've ever done before. Building to a climax, you start pumping harder and harder...
And then nothing happens.
The dildo doesn't have a hole on it at the end. It's not a penis, it's just rubber shaped like a penis that happens to give you the same senses as if you massaged your own. It's incapable of cumming, and, to your horror and delight, it's still massively erect and sending wave after wave of pleasure to your brain.
Paralyzed by the feelings, you lie down on your bed and wait for your strange new penis to go limp. However, it never does. It stays the exact same length it was (a hefty 14 inches) when you bought it. It still feels amazing too. You need to cum in the worst way but after a few minutes you grow used enough to the pleasure to think up a plan.