A few hours after Mark left his home, his younger brother, Felix, came in looking for him. Mark had been so enthusiastic about his horse transformation that he forgot his family will be coming at his place that day for his little brother, David's birthday.
Despite Mark was the older sibling, Felix was considered the more mature and responsible one among them. He liked to take the organizer role, the guy who plans everything parties, trips, important decisions, etc. He didn't ask for it, his family recognized his capabilities and he liked to help them.
However, unlike his brother Mark, Felix was too serious for dreaming and having fantasies, he didn't believe in ghosts, much less in magic stuff like turning into an animal. He even had made fun of Mark when he confessed him about his secret desire to become a horse. Who will want to be a dirty barn animal when you can do much more as a successful man? He told his brother in that occasion.
That day, Felix was looking for his big brother, who suspiciously didn't seem to be around. He knew Mark had a bad habit to let door open, including his home or someone else's main door at night. He tried to phone him, but he immediately realized that Mark's phone was at his kitchen table. Where was he? He kept wondering while he looked in each room.
When he got tired of looking for Mark, he decided to keep on his preparations for David's birthday. He started to pump balloons, setting the table and some other stuff. He was unpacking some gifts when he found it. The same vial that Mark had recently drunk was there with its shiny liquid inside almost tempting Felix to drink it.
He took it, opened it and sniffed it. It was a smell that he can't relate to anything. Was it liquor? Or a drug? Was Mark on drugs now? If mom or dad knew about it... Feliz was thinking while he kept watching at the vial in his hands. After a brief moment, he tried to ignore it and kept going on with his decorations but from time to time he can't avoid looking at it.
That was one of his weaknesses, being nosy. If his parents find out about it before, he would never forgive himself. He need to be sure and he can think of only one way to know it. He placed the birthday cake at the center of the table and everything was in it's place. It was the moment.
Without thinking too much about it, he took the vial, he sniffed it once last time to confirm it smelled like nothing he can recognize and then he engulfed it. "Hmmm, it tastes weird. I'm still not sure if it's a drug, but..." He was saying out loud when he felt something weird happening on his inside.
He held his stomach in pain. Was it toxic? Maybe he had just drunk shampoo or something like that? Why didn't he think about it before? He kept thinking while he came out to the back yard. He needed some fresh air and maybe to call an ambulance.
However, when he tried to take the phone in his pants, he realized he had suddenly got bloated. His legs were so swollen right now that he can put a finger in his pocket. Was he getting an allergic reaction? He was now so desperate that he started to cry for help. But no one came until it was too late.
Felix was changing despite he didn't want to believe it. Just like Mark, his skin was being taken over by reddish fur, with occasional white spots on his limbs, butt, and back. His hands got bigger and his fingernails fell out, being replaced by thick solid hooves, which were enormous in size. His arms became muscular and bigger every passing second. His torso grew muscles too, revealing a very large and barrel-like gut that ripped his navy blue dress shirt.
His legs, already bigger and stronger, grew larger muscles that matches his new fore legs. His feet grew bigger as his toenails fell out, also being replaced by thick black solid hooves, also enormous in size. Long gone were his new black dress shoes.
He can't believe it what he was looking, the hooves, the muscles, he was becoming into a horse, just like Mark's fantasy! But that was his brother's dream not his. He stopped thinking about it when he began to feel a pain in his spine. A long tail was pushing out and growing white hair all the way to the tip, which now was hiding his already swollen donut-like horse anus. At the same time, a long white mane started to grow on his back replacing the hair he used to have on his head.
He was completely mortified of getting the body of a beast, but it was just getting worse when he started to feel changes in his private parts. "No, no, no, no!! Please stop it!!" He was crying while he watched his cock and balls, getting covered in a black leathery skin and tripling in size, until his balls become the size of grapefruits and his cock becoming the size of a bottle rocket. Tears were running down his cheeks while he watched his now bestial genitalia getting hidden inside a dark smelly sheath made from his former foreskin and attaching to his abdomen.
Feliz was almost a full horse right now, except for his head but that didn't last longer. He felt a sharp pain in his skull as it was forced to push out into a horse muzzle, with his nose becoming a big horse snout. His teeth fell out, being replaced by big and square horse teeth. His tongue grew longer and heavier. His ears long and pointed. His eyes turned brownish and his pupils lost their humanity look.
Finally, his already big body got even bigger as his body grew massively in size and mass as he got hundreds of pounds, until he was as big as a championship shire stud. Then, the pain faded away but he kept panting for a few more minutes. He instinctively had adopted a four leg stance while his barrel-like chest pumped air in and out. It seems that his tail had gotten a life of its own, because it was moving side to side without him knowing about it.
After stopping panting, he began to panic and neigh as crazy, he moved his body like a mad animal until he crashed with Mark's fence. It got broken and Felix ran away towards the forest without knowing what will happen to him.