Disclaimer: I do not own Sonic the Hedgehog, or any of its associated characters and lore.
Under the Eggman Empire
The Prowess of the Twin-tailed Fox
The Battle Fortress, Earth, 3234 Post-Xenomorph Event
As the mobile headquarters of the Battle Kukku Empire flied over Cocoa Island and the surrounding Great Pacific Ocean, a battle was being fought against a single attacker. Having spent a considerable amount of time surveying Cocoa Island and its neighboring islands in search of the Chaos Emeralds that were said to be located there, the Battle Birds had been driven to the Battle Fortress after the attacker arrived and made his move with numerous gadgetries and his own natural skills.
After so many soldiers hand fallen, only the chief scientist, Doctor Fukurokov, the fifteenth Mobian rooster to bear the name of Great Battle Kukku, and the latter's son who was aptly nicknamed "Speedy" were left to fight the invader, and while they all sweated in anticipation of what was to come, they had no intention of fleeing.
"Be ready, both of you." The Great Battle Kukku said to his two remaining subordinates. "No matter what happens, we'll send the little freak's lifeless body back to his overweight fool of a master."
At that moment, an explosion occurred close to them which gave their foe, a young fox with two tails and wearing the uniform of the Eggman Army, an opening to press his attack on the three leaders.
"Prepare to do battle with the finest doctor that ever came out of the Battle Kukku Imperial Academy, you filthy sycophant!" Fukurokov proclaimed as he ran out, bombs in hand, but was quickly knocked out with a hit to the head by the fox assailant, who took the owl doctor's bombs for his own.
It was then that Speedy made his move as he flew straight for the serious looking youth. "It's time you know your place, you non-bird flyer!"
Instead of responding in words, the fox threw Fukurokov's bombs at the crown prince, who fell to the ground unconscious from the resulting explosion.
"Speedy!" The Lord of the Battle Birds cried out in concern for his heir before turning to the Eggman Soldier angrily and quickly ran straight to the fox to retaliate.
However, it was then that the young vulpine jumped, propelled his tails to enable flight for himself, and he began to punch the Great Battle Kukku in the face hard. Although the latter attempted to respond in kind, the fox was too fast, and he was finally defeated when the Soldier slammed his whole body in the older rooster's torso. As the Great Battle Kukku tried to get up from the floor, he saw that the fox was pointing his red arm cannon down at him, forcing him to hold still a little while longer at least.
"Great Battle Kukku XV." The fox addressed his beaten enemy before adding. "I, Miles Prower, on behalf of the Eggman Empire, have a message for you. Surrender to the stronger Empire or suffer the consequences."
Central Office, Citadel Eggman, Metropolis, West Side Island
At the egg-shaped administrative building/fortress in the heart of the Eggman Empire's capital city, the titular Eggman himself, Doctor Ivo Robotnik, sat in his personal office signing away some documents until he received a beep from his intercom.
"Doctor Eggman, sir, I have Specialist Prower on line three." A feminine, yet clearly robotic, voice informed him.
Liking the sound of that, Eggman pressed the intercom to reply. "I was just wondering about our boy, Receptionik. Put him through on the monitor."
On a nearby viewing screen, Eggman then saw his personally trained Soldier holding the leader of the Battle Kukku Empire at gunpoint.
"Your Excellency, I am pleased to report that the Great Battle Kukku XV is willing to discuss terms of surrender to you." The Mobian fox with a proud expression.
Showing an eager smile himself, the Doctor said. "Well done, Tails, well done. I shall deploy a detachment of the Eggman Fleet right away to reinforce your position." Turning to the scowling Battle Kukku, he then said. "No need to look so defiant, Kukku, this outcome was to be expected. In any event, once your forces are properly adjusted for life in the Eggman Empire, I shall direct your area of operations to Midesta, where I believe your precious Babylon Garden is located." Allowing himself a laugh at Kukku's look of sheer surprise at the news, Eggman added. "I figured you would find that interesting to say the least. I trust there will be no trouble from you moving forward?"
Although disgruntled, the Great Battle Kukku XV nonetheless held his head down as he replied. "I will need to address my troops, sir, but I can assure you that any insurrection on their part will be handled in due course."
"Excellent." Turning to Tails, Eggman than said. "You are to be rewarded when you return to Metropolis, my protégé. And once we have the Chaos Emeralds, we shall see of obtaining that which your heart itself desires."
Showing a grateful look in his eyes, Tails held his head down and said back. "I'm looking forward to it, sir. Prower out."
With the communication over, Eggman thought of how he captured the then newborn Tails' parents and attempted to Roboticize them both before the two were surprisingly abducted by an alien spacecraft orbiting Earth for a brief period and were then never seen again. It was quite amazing how placing Tails in the right environment and saying the right promises would engender such loyalty in the youth.
The concept commonly referred to as the Assistant Tails Alternate Universe, which is about Doctor Eggman "recruiting Tails to his cause" instead of Sonic, struck me as interesting, so I decided to take a shot, but some discussions on SpaceBattles and reminiscing about Sonic media featuring multiple people working for Eggman got me thinking of making a series of how many characters would function under the Eggman Empire's rule. I wish I could make a real story out of this, but I have many others I want to work on still. If I do see a good depiction, I will be sure to recommend it, though.
Speaking of Tails, I also figured that the idea of him being born with only one tail would make for an interesting story of its own as well since without his second tail, he probably would not directly accompany Sonic and his friends on their adventures without the Flight, Speed, or Power moves needed to defend himself. I can imagine that he could build his own mech from being even more devoted to his scientific pursuits, but at least Eggman is classified as a Power Type even without his own machines. And yes, he could devote himself to being as fast as any Speed Type, I am just offering a probable idea.