There were many horses in Animalia now. There were even a few donkeys. But mules were a different beast. Since tractors and trucks have become more common for agriculture and transport, mules were not as common as they used to be. As such, there weren't any mules in Lyre when the first outbreak took place. And there didn't look to be any coming for a long time as the horse and donkey Animalians were more interested in others of their own species than each other. However, there were still some dedicated breeders of mules in the country and Horizon Labs was able to get in touch with one of them when Project Chimera was relaunched.
It took a while to transport the mules to Lyre on account of them having to be driven from Nebraska. Eventually, the semi truck and trailer did arrive at Horizon Labs. One of the scientists came out to talk with the trucker. "I'm glad to see that you've made it," said the scientist, "It must have been an awfully long drive."
"Well, I don't think these two woulda liked to fly here," said the trucker, "Name's Magnus, but everyone calls me Big Mag." He definitely fit the image those names implied. He was almost 7 feet tall, broad-shouldered, barrel-chested, and certainly 'larger than life'.
"I see," said the scientist, "Are these mules yours?"
"No, a friend owned them," said Big Mag, "I was just doin' him a favor and brought 'em here." He walked to the trailer's back and opened the door. "I reckon they'd like to stretch their legs now," he said.
"Well, of course. They may as well get comfortable before we- Holy mammoth!" The scientist gawked at the size of the mules. One mule was jet black with a star of white on his forehead and stood 16 hands tall. The other one was a hand shorter and bay-colored. Both of them had feathering on their legs and thick, muscular bodies. If it weren't for their long ears, the shape of their heads, and their short manes, they could easily be mistaken for draft horses.
"I'm guessin' they didn't fill you in on what kinda mules they are," said Big Mag.
"Well, they're not what I imagine when I think of mules," said the scientist.
"Well, mules come in all sorts of shapes and sizes, depending on what breed their dam was," said Big Mag.
"I take it these two were bred from a Clydesdale?" asked the scientist.
"Ardennes, actually," said Big Mag, "Their mother was a rare black mare and blacklisted from registration. Doesn't mean she couldn't breed good mules though."
"Oh, they're siblings then?" asked the scientist.
"Yep, the big guy's Olly and that's his sister, Nelly," said Big Mag.
"They must dwarf their father by a lot," said the scientist.
"Not really," said Big Mag, "They were sired by an American mammoth donkey. They can get pretty big too."
"Well, fortunately we're equipped to handle large equine Animalians," said the scientist.
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The two mules were taken to one of the larger test rooms. It had been mainly used for the uplifting of big cats. In fact, Heracles had been uplifted here. The room had been scrubbed so the mules wouldn't smell the predator's scent. Considering the two of them needed plenty of room, the table had been removed. And as soon as they were given their shots, the lab assistants cleared out.
As with many horses before them, the mules' transformations were pretty much standard at this point. Their huge barrel-like torsos crunched as they reformed into humanoid torsos. Their front hooves cracked as they split and started forming into fingers. As their legs got more reshaped into bipedal form, they were standing up on them. Olly's shoulders were expanding to a very impressive width as he was building up muscles fit for a super heavyweight bodybuilder. Nelly was also bulking up to that size class, but her figure was still on the feminine side, especially with her new breasts growing larger.
When the transformation was complete, the two mules looked a lot like the other horse Animalians. Their donkey-like head and ears were the main indicator of them being hybrids. And their muscles clearly showed that they were as strong as mules were supposed to be.
Olly looked at Nelly and asked, "Nelly, is that still you?"
"It is, Olly," said Nelly, "But you look so different."
"So do you," said Olly, looking over Nelly's altered form. Her large bare breasts particularly caught his attention. Before long, his long stallionhood was starting to slide out of his sheathe. That produced an emotion the mules never experienced before: embarrassment. Olly blushed and tried to cover his junk while Nelly did her best to look away. While they could both see they were very attractive members of the opposite sex, a deeply-rooted instinct was telling them that they were not potential mates. Perhaps this instinct was an inherent part of animals to prevent inbreeding or maybe their new human intelligence was telling them precisely why it was wrong to mate with their sibling.
Thankfully, they were able to turn their attention away from each other when a voice on the intercom said, "Pardon me for interrupting, but perhaps it is time for you two to begin your introduction into Animalian society, along with a few tests." The mule siblings weren't quite sure what that meant, but they were grateful to not have to focus on each other in their vulnerable states.