He decided to note it for now and question it further later and just appreciate the fact that he was probably the only man in the world with a sheathe. Besides, his thoughts started to turn elsewhere with his face buried in the horses genitals. He couldn't stand it any longer, the smell of the horse was getting him extremely aroused, and he started licking the horse's pussy with reckless abandon, licking up and down both of the outside lips and taking a moment to lick inside the middle of it's vagina before tongue-fucking the hole, occasionally moving upwards to tongue the horse's puffed out anus.
Finally he couldn't stand it anymore and had to put his penis inside the horse. He frantically searched for something to give him a boost and this time found a sizable rock to stand on. Figuring it would work, he hurriedly dragged it to the horse's backside, where he dropped it on the ground, climbed on top of and furiously inserted himself into the horse and started humping. He lasted a little longer this time and the horse seemed to notice it more too, flagging its tail to the side and looking back at him most of the time now. When he finished, he slumped forward again, smiled, and patted the horse on the rear a few times. "Good girl." He said drunkenly to the horse while patting. After a half a minute his cock fell out of the horse and he got off the rock and slowly made his way back onto the saddle. In doing so though he forgot to put his pants back on and now sat on the horse half-naked, his pants forgotten in the bushes back where they had just had sex. His sheathe was now on full display to anyone who passed by, though not many people used that trail, so it shouldn't have been an issue. He didn't even notice and just rode the horse with a smile on his face and a light in his eyes as he looked forward. As he rode he could feel his cock twitching now and then, mostly still but stirring a little now and then. After a while it started to throb though and he stopped looking forward to stare questioningly at his crotch, wondering what was happening. His cock was filling with blood and leaving the sheathe again. However, more left the sheathe this time than last time. Since he had last seen his cock it had put on at least two inches in length and now stood at an impressive 8 inches. "Wow I guess my changes aren't done" he said to no one in particular as he stared at his now firm and lengthy cock. He wondered how far his changes would go. He already had a sheathe and a larger than average cock. Would he get a full horse cock or would this be it? He certainly didn't see why he wouldn't go all the way to having a horse clock if it was still changing.
The more he thought about the fact the more it would make sense. He had fucked the mare twice.
Only stallions are supposed to fuck mares. Stallions have sheathes for their horse cocks. So he was a stallion and he had a sheathe. And a horse cock. Right? He thought to himself. Frowning, he looked down at his cock that had stirred out of his still pink sheathe. It was a little bit bigger, but it wasn't a stallions cock. He was a little disappointed. He really wanted to have one now. He wanted to full how it was to fuck a mare with a full sized horse cock, and it would go so nicely with his sheathe...
It made so much sense that he couldn't think of why he didn't have one at the moment. He certainly should. Frustrated, he thought of how he had gotten this far. He had done nothing much interesting the last few days, and hadn't noticed anything until he was at the pond. "Maybe it was something in the water!" He exclaimed aloud. But nothing has still changed until he had sex with the horse for the first time. Maybe that was the trigger?
It was the only thing he could think of and frankly he hoped it was. Just the thought of something g he enjoyed so much giving him the key to something he should rightfully have made him happy with glee.
With high hopes, he again found a nearby log, placed it behind his mare, and, 8-inch human cock fully erect sticking out of his humanly inhuman sheathe, lined himself up with the horse's vaginal opening and pushed himself into the horse for the third time that day. He didn't feel anything right away. But he didn't feel like thrusting this time. He was already pretty tired from having gone twice already, and decided to instead leave his cock in place in the horse, enjoying the feeling of warmth wetness and the surrounding flesh.
The horse didn't seem to mind much and let him do his thing, seemingly content to just stand there, occasionally bending down to smell some grass or eat it.
After five minutes of nothing, Jason pulled his still human penis out of the mare, disappointed.
He felt rather down and decided to get back on the horse with his still human-but-larger-than-average-sized-cock-with-a-sheathe in tow. He had made up his mind to just head home and figure out the rest when he got there. His penis softened up and slowly went back into his sheathe.
As he rode, he hunched his shoulders and looked at the back of the horse's neck. He really wanted to finish his cock's transformation to a stallion's penis but he didn't know how. He thought being some halfway thing like this would just make him a freak, but having the whole stallion cock and balls set with sheathe, that would really be something.
While he thought this, his penis was simmering in its home, covered with the mares sexual juices. The virus he had been infected with and the mare was a host to had the effect of changing humans to match the animals they mated with, while also skyrocketing their lust at the moment of infection. The virus concentrated itself in sexual fluids and spread this way.
Jason's penis was covered in tons of horse DNA and was coated in sexual fluids from the horse. Combined with the virus already acting in his system, this was a powerful cause for change.
The first thing he felt was a slight throbbing in his penis. He first thought he was just getting horny again and was about to get off the horse to give it another go, when something different happened. After his cock his risen to full mast, he noticed that with his heartbeat coursing fresh blood through it, a section in the middle of his cock started to puff out.
His eyes flew open wide and he immediately fixed his posture to stare better at his cock. "This must be a medial ring!" He thought with increasing excitement. After a minute it was finished and he now had a thick spot in the middle of his cock that signified his ongoing change into having a horse's penis. With high hopes, he kept staring, waiting for anything else to change. He was not disappointed. Soon after the medial ring had finished growing, he felt the head of his cock pulling in all directions. It was like someone was pushing hard directly on the tip of it and the flesh had nowhere to go but in all directions outward. This growth continued for a minute too until he had a flat clock head with bumpy ridges around the side.
"This is it! Mostly there!" Thought Jason. "Now I'm just missing the color right color and size..."
He was rewarded with the right color soon after the head had finished. Before his eyes, various parts of his penis started to turn black, while the parts that remained pink took on a lighter hue. He was amazed with the changes that had happened so far, but the one he had wanted the most, to have a cock the size of a stallion's, was still eluding him. What he had now was a large but still human sized horse penis complete with sheathe attached to his groin. Though he waited for a few more minutes, nothing further happened.