Jeff was a sort of masochist. He always got himself off with the frightening idea of turning into a donkey, and having to remain one for the rest of his life. He had these fantasies, not because they were "fun," and not because he wanted to be a donkey. No. Quite the opposite. He had these fantasies because it was horrifying, frightening, and hopeless. He had these fantasies because he would be rendered helpless. Especially having hooves to replace his hands, and calling his arms "legs." That was always the most frightening thing of all, and was a major point that got him off.
He fantasized about donkeys specifically, because they are large creatures. They are very undignified unlike horses. They have taller, more obnoxious and broader ears than horses. They have an irritating braying noise they make, and to be stuck with such a voice with the inability to speak, always got him off as well. Hooves would render his ability to play video games, turn pages in a book, use cell phones, or play baseball impossible.
Jeff had perfect teeth, and he knows it. Tonhave his human teeth replaced with the large, blocky, yellow teeth of a donkey who can never brush it's teeth was not just a further indignity, it was downright disgusting. To be only be stuck eating someone's lawn or yellow hay, rather than pizza, hoagies, or ice cream ever again was also a high point. So was the idea of never using a toilet; the outside would be his toilet with no privacy at all.
He would never shower, and would always have the stench of a farm animal about him. A tail that pokes out of a huge rearend that sticks up. An enormous black, smelly dick, with huge black, rubbery balls that wobble around with every movement.
He would also add the inability to keep himself from mounting a Jenny. Horrified at the very idea. He would have a small piece of actual donkey brain that would take over his body while leaving his human mind completely intact and fully aware of his actions any time a Jenny was in heat. The thought of mounting a Jenny with no privacy, with his front legs and hooves forever struggling to find purchase on her shoulders. He would have his large, smelly cock uncontrollably buried deep into a Jenny"s large, dark, fishy hole. And it would remain there until he procreated with her. What would drive him to cum in this position, would he his ultimate and overwhelming desire to just have hands. He would imagine himself trying to will his hooves to "open." To grow fingers so he could at least grip something. But his hard, useless hooves would remain forever closed with no possibility of ever gaining his fingers back. He would have "phantam fingers" and they would "feel" perpetually balled up and trapped inside his hooves. If only he could break those hooves and free his fingers! But he cant. He has no fingers, and he's trapped. Forever just like that.
Jeff also has perfect vision. While he'd imagine himself having decent vision as a donkey, it would be sufficiently different. To the point of constant irritation to him. It would be extreme binocular vision. Very limited amount of colors. The world would never look as vibrant without the ability to see the color red. It would be so extreme, as to render his ability to read impossible, even if he just had fingers to turn pages of a book.
His life would be irritatingly boring. Nobody to talk to, and no way to communicate, except for the most basic wats if communicating. And only with members of his own kind.
And the idea of seeing a small group of donkeys milling about, and "joining them." Being sticking looking like and doing what they do. In their world, which would now be his. Looking at a road or people walking away, going about their business to see things he would never see again, and do things he would never do again. Leaving him behind in an empty field with other donkeys he couldn't even talk to, even if they could understand speech. His loneliness and being "left out" was another thing that left him horny.
And so, Jeff has this chronivac thing that would be able to do that to him. The very idea made him furiously horny. He used to get off on the idea of slowly turning into a donkey, but lately, the idea of instantly becoming a donkey without any chance to say goodbye to various human features has been his "thing." And more to the point, his consciousness and soul trading places with the consciousness and would of a natural-born donkey. An especially disgusting, foul smelling one with green slimy grass stuck between it's nuclear orange teeth. Overweight, and middle-aged already. With mud and manure already cracked in the crevices of both of its hooves. Producing thick, yellow, donkey seed that would only be capable of producing more natural-born donkey, and never a human baby.