As you go through the list of creatures, which you admit is quite vast, you decide on something simple, the majestic horse. A stallion at that. You left all the senses on because you know (or at least think you know) nothings going to happen. With that said and done you hit enter.
Nothing happened at first, nor through the rest of the day. That night you went to sleep knowing the machine was a dud.
Once you woke the next morning you were greeted with a nightmarish sight. The lower end of your body was fused into the ass of a living, breathing white stallion. The horse didn't seem to mind the deformed human fused to its rear, in fact he seemed to enjoy it by the way his hard cock swung between his legs. The stallion was pure white, was well endowed and most certainly a muscular build. You on the other hand didn't look to good. Your neck down was scrawny and withered creating a some what tube like shape. You had no pelvis or lower of your own but had your body instead melded between the plump, sweaty cheeks of the horse. Your skin began to take on a black like hue but was most pronounced around your now puffy, tight, pulsating lips. You had no hair besides those that were now your new owners. You seemed to lack any organs but had blood circulating by the horse's heart and a fleshy tunnel like structure that went from your mouth to its previous asshole.
This was terrible! How could you risk your lively hood on a stupid mysterious machine, but it was too late now. The damage was done.
With that thought done you felt the horse began to push. You watched the tail raise and a lump move through your slender body. A mass you would know too well. Before it even reached you puckered lips, you tasted it on the the heavy burst of flatulence that escaped your mouth, you tasted shit. You reached for the lump with you tiny noodle arms to try and stop it, but your arms were to short and were seeming to slowly melt into your mass. Many more feature began to shortly follow as the log of shit crept toward your face. You felt your body behind the turd recede back into the horse, who began to ejaculate to the anal stimulation.
Finally the brown mass met your lips. The sensation of the log spreading you lips caused the features of your face to melt away until you were indistinguishable from a horses anus. The taste was suddenly less horrid and the sensation felt normal. As the shit pushed outwards you, you squeezed and broke the mass into balls as if you've done this a thousand times before. Finally it was all done. You were a horses anus with a conscience, what ever was your body now laid as a steaming pile of horse dung on the ground and you felt kinda happy. This was a new start, things can only get better.