Six weeks ago...
Sonic was going for his usual run. He had just finished eating his breakfast of chili dogs, and had just zoomed out of the house, giving his friend a high five.
He zoomed down the running path. Everything was going normally.
He hit the bar at full speed.
Sonic was knocked unconscious from the impact against a bar that wasn't normally there, and fell into a net. This net was lifted into the air and carried away.
Several hours later, Sonic woke up with a massive pain in his head. He looked around where he was.
He saw a room filled with strange torture devices. Whips, collars, and...were those dildos?
The door before him slid open as a large figure stepped inside, the door closing behind him.
"Hello, Sonic"
"Eggman!"
Sonic zoomed at Eggman, but his ankle caught on something. Looking down, he saw a chain attached to his ankle and the wall on the far side from the door.
"What do you want from me, Egg Head?"
"Experimentation, my dear hedgehog" the human responded.
"Whatever it is, it won't work!"
Eggman gave a dark grin. "Don't be so sure about that"
Eggman walked over to a nearby wall and punched in a long and complicated code on the number pad. When he finished, a tray slid out. Sitting on the tray was...
A pink Master Emerald? What on Mobius?
"You see, Sonic..." the man began. "...This is the Lagneía Emerald!"
"And what's it supposed to be? Another Chaos?" Sonic jokingly asked.
"No. This allows me to alter your body in any sexual way I please" his grin persisted.
"Wh-what?" Sonic couldn't believe what he'd just heard.
"Now, let's see just how well this works! Remove his clothing!"
A pair of mechanical claws descended from above, grabbing at Sonic. Instead of simply removing the clothing, however, it grabbed at it and tore it off.
This revealed Sonic's manhood. It was average, sitting at six inches, and only an inch in thickness. He'd always wished it were bigger but hadn't done anything about it.
Eggman's dark grin persisted. "Let's see just how well this works" He moved the gem to point at Sonic. A pink beam blasted out of it, hitting Sonic directly.
Sonic's manhood began to grow erect as the energy spread, focusing on his groin. When it reached its full size, it started to gain length.
"Ah, perfect!" The human shouted.
"A-ah! S-stop it!"
"Oh, but the fun has only just begun!"
The beam fired again. This time, Sonic's member and balls shrunk down, until they faded into his crotch, becoming nonexistent. Instead, a tingling feeling formed as the same general area began to split. The slit grew longer and deeper as he gained a fully formed female vagina. The last thing to appear was a sensitive clit.
Sonic whimpered. "What are you doing?"
"I've spent a long time feeling pent up, hedgehog, and I think now would be a good way to let myself go"
"No...no! NO!" Sonic was screaming in horrified protest as Eggman undressed himself.
The human walked over to the much smaller hedgehog and picked him- better make it HER up. His cock was already fully erect and aimed.
"Have fun losing it!" With that charming shout, Sonic was slammed down. The new cuntboy screamed as her new hymen was forcibly torn and her body painfully formed around the large member inside.
The present Sonic decided to spare us the details of what happened next, thankfully.
A few days later, and Eggman had come in for the fourth time in a row. Each time he tried some new sexual fantasy. That was then reason Sonic was as she is now.
Several weeks later, Sonic finally found her chance. She had been given then special clothes so she could sleep comfortably.
She had the clever idea of hiding in her shirt when Eggman had come around for that day. She did, and it miraculously fooled him! He took the shirt out and then Sonic broke free and ran off.
Due to her abnormal endowments, she was slower. She wasn't able to dodge all the fire, or escape at the speed of sound. She did, however, manage to escape. She ran out into the surrounding forest and managed to find her way to New Mobotropolis.
That was about when Tails had found her. And he knows the rest.