This is DR. Theropuff Origin story, this was originally a comic, but I felt like this would work best as a Story.

A Doctor’s Origin.

It was another quiet day at the lab, the new intern, Reyo Theropuff, a somewhat young but oddly short rabbit, was assigned to do some cleaning in one of the rooms with experiments gone wrong.
After getting the cleaning materials, Reyo tried to carry it to the room, but due to his size, he found himself having quite a difficult time even walking around with the broom.

After a hell of a time getting in the room, Reyo discovered that the room door was locked in the inside somehow. But since nobody was around to help him, Reyo took a look around, and found a small vent which seemed to be big enough for him to fit through, so he did just that.

After managing to get inside the room, Reyo went to try opening the door, only to turn around and see a near spotless room, with a gigantic glass tube, filled with a red liquid, and a metal tube linking it to an odd machine.

Reyo was curious about the gigantic glass tube, after all, it was the only thing there, and why would they tell him to clean a spotless room?
With a curious touch, Reyo touched the tube, and a quite annoying ping ringed through the room, which made Reyo a bit confused on why it did that.

In a few seconds, a powerful aura came from the inside the tube, and Reyo immediately ran back into the door, trying to find the lock. But a voice came into his mind:

“Mortal…”

Reyo panicked, trying to find the lock as quickly as possible, but there wasn’t one, almost like this door wasn’t a door at all.

“Mortal… Help me…”

Reyo turned around, despite being scared, His curiosity spoke louder.

“Who… are you?”
“My name do not matter in this situation… Help me”
“How can I help you?”
“Free me from this… purgatory”

Reyo went closer to the Glass tube, was the red liquid alive?

“What are you?”
“I was the powerful dragon known as Trimenty.”

Reyo felt a shock hearing that name, Trimenty had the legacy of the most powerful Dragon, aside from the royal dragon family, people thought he has been assassinated long ago, and his body was long gone.

“How… How are you alive? The legend-”
“The Legend is something the royal family came up to not discuss my… departure”
“But… How are you…”

Reyo couldn’t believe that one of the most powerful dragon of the land, who thought to be dead, was somehow right in front of him.
“But how are you, like this?”

The silence echoed through the room, Reyo looked around, and noticed how the machine had several empty bottles on the side of it.

“I was captured, tortured, and the researchers took what was left of my body, and I was preserved in this prison.”
“How long have you been here?”
“Far too long, my bones are rubber at this point.”

Reyo stared at the tube, something about this story was confusing him.

“You asked for my help, but even if I break this tube, how will you…”
“Of course I won’t be able to do much, that’s why I offer you a deal.”

The room fans turned on by themselves, a few researchers appeared and looked through the little glass window, but none of them seemed to be paying attention to Reyo, they seemed to be interested in the machine, which turned on with the fans.
Trimenty was forced to scream, as his power was being pulled away from his body into a bottle of pure magic.
Reyo felt a sense of justice come over him, why were they doing that to someone who was chopped into bits?

A few minutes few by, and the machine, fans and lights went out, with the researchers leaving quickly after, some carrying a few of the bottles.
Reyo looked at the tube, and the treating aura from before was now almost gone.

“What do you propose?”
“As you can see… My situation isn’t the best…”
“And this is why I want to hear your proposition”
“I propose, that you allow me to merge with your body.”

Reyo took a step-back, before continuing.

“That’s quite a big thing to ask.”
“I know… you’re not the first one who I ask for such a thing.”
“Wouldn’t I lose my body if I allowed you to merge with it?”
“Not at all, I could even do some… improvements to your body.”

As the two continued talking, footsteps could be heard, coming closer.
However, this time, it seems like it's only one.

“Why would you want this? What would you gain from doing this?”
“Freedom, at this point anything is better than to continue to suffer like this.”
“But, how would all your magic and power be able to fit my small body?”
“Simple, I’ll turn you into a dragon.”
“What?!”

The footsteps grew closer, seems like whoever was coming was carrying something big.

“Of course, I don’t intend to turn you into a complete dragon, as your rabbit form would be more beneficial to me overall.”
“So you intend me to become a hybrid?”
“Essentially, yes”

Reyo thought for a moment, this would change quite a bit of his life, and it didn’t seem fair to see a somewhat idol of his give him something so special, with nothing in return.

“Alright, I accept your condition.”
“Perfect.”
“With one condition”
“And what would that be?”

Reyo took a deep-breath before continuing.

“I want to you fully merge with me, not just fiscally”

Silence, before the footsteps which were coming closer, eventually went right through into another room.

“Is that so?”
“Yes”
“Fine by me, now, how do you plan to free me?”

Reyo took a look for something to break the glass, but there was nothing, the room was really empty of anything sharp, but the pipe connected to the machine has loose screws, so, Reyo grabbed the pipe, and took the screws off, unlocking the pipe, which he used to turn right into him.
In a few seconds, the now loose pipe started pulling Trimenty into the floor tiles.
It took about 30 minutes, then, Trimenty asked Reyo to stand on top of him, which wasn’t difficult, since he was mostly in the entire floor.

“Are you ready?”
“Yeah”

In a single swoop, Trimenty started climbing Reyo’s leg, and entered his body by all the available holes, which made Reyo quite uncomfortable.
Quickly, his body was becoming filled with Trimenty, and he could barely move.

“Your body is indeed quite small, I’m not even inside you, and you’re already full, seems like I’ll need to start modifying your body a bit sooner…”

After saying that, Reyo’s skinny limbs quickly grew bigger, to a not quite, but somewhat muscular appearance, and a red large dragon tail came out.
Reyo was currently trying to stay conscious during the process, but his mind kept fading in and out as Trimenty entered more and more into his body.
As he struggled, he eventually caved in, and passed out, until waking up in the lab, hearing Trimenty calling his name, as he opened his eyes, he felt bigger, even accidentally hitting his head on the cealing.

“Sorry to wake you up, but if you truly want to merge with my soul as well, then you need to be awake.”
“Alright, but how will I-”

Trimenty didn’t waste time, and tossed himself into Reyo’s soul right away.
Reyo felt almost as if he was becoming someone else, soon, their souls finally combined, and they were one.
But somehow, he was still somewhat Reyo, so he kept his name, but his powerful body wouldn’t fit to be called by just his first name, so, he decided that he shall be called DR. Theropuff for now on.

In a single swirl of his dragon finger, he changed his form from a massive dragon, to a still small but stronger Rabbit, with red details in his white fur, and a somewhat large tail.

Despite being naked, DR. Theropuff opened the door of the room, and left the lab, to never come back, and start his own legacy.