Spike had to do some more swallowing. "And one of those outcomes has me crashing through the roof?"
The princess settled back onto the edge of the pond at Spike's big clawed feet. "The percentage on that one's honestly pretty small. The portal should work the same way as before, shunting you to exit at the transverse center of the nodal complex on that side. So once you hop in, you should drop out just above her bed, and she'll be standing right there with her magic to catch you."
"If I don't come out as a giant dragon." He looked at the placid surface.
She shrugged. "That's another one with a pretty small probability." Sighing, she went back to writing in her book. "What's most likely is that you come out the other side exactly as you went in four days ago."
A clearing of throat drew his glance to the other Spike, gliding over the pond with a few flaps of his wings. "I'm afraid I have to disagree with you there, Twilight."
"Really?" The princess glanced away from the book. "You have more data?" Her eyes lit up. "Do I get to do another batch of calculations?"
The smaller dragon shook his head. "You've just gotta trust the magic."
She blinked. "What in the bright blue name of Equestria do you mean by that?"
With a sigh and another couple flaps, the other Spike rose to a position where he was looking Spike right in the eye. "Remember what I said earlier about the friendship you and Twilight have, the love and the connection that make magic happen? Well, magic changed you once in your world, brought you here to change you again, and I'm convinced it'll take all the old and the new and change you even more when you go back." He reached out and touched a claw to Spike's nose. "You can tell me about it next time you visit."
The princess's mouth squinched up on one side. "That's not exactly scientific."
"Exactly." The smaller dragon didn't budge his gaze from Spike's. "It's magic." He hovered back a bit. "I'll see you around."
A lump was forming in Spike's throat. "Thank you," he forced out around it. Then he looked down, nodded to a confused-looking Princess Twilight, and stepped off the grass into the water.
Strangely enough, he didn't fall, or at least he didn't feel like he was falling. The colors and shapes rushed over him the same way they had on his first trip through the portal, giving him the impression that he was moving forward, and the almost musical sounds seemed to start in front of him and flow past.
They seemed different, though, the colors and the shapes and the sounds, still strange and jarring, sure, but gentler, maybe? Less like they were poking him and laughing and more like they were petting him and cooing. Like whatever force it was swirling and crashing around him was trying to reassure him...
Wishing he could find his eyes so he could close them the way he usually did, Spike still sent his silent 'thank you' out to the power that had given him more than a dozen years of being able to talk to Twilight and understand her—well, as much as anyone could understand her, of course. No matter what happened next, he would always be grateful for—
The shapes and colors all froze at once, flattened, and cracked like ice, and before he could draw another breath, Spike found himself looking down on Twilight's familiar room, her bed just below him. Three human scents tickled his nose in a way that seemed more canine to him than draconic, and Sunset's cry of "There he is!" perked ears that felt like proper ears along with Fluttershy's gasp and Twilight's "What the—?"
Then the stroke of her power was prickling his fur—
Fur instead of scales! Yes!
But it somehow seemed to be more fur than he remembered, and he whumped pretty heavily into her coverlet and blankets, sank into them more deeply than he ever did when she would let him up on the bed.
"Spike!" he heard Twilight shout. "Is...is that you?"
"Twilight?" he said, his jaw moving and the word coming out in a voice a little deeper than he was used to. Steadying his paws against the squishiness of the bedclothes and mattress beneath him—they were paws, he could feel, but did they seem wider?—he pushed himself up, turned toward her on his right, saw her beautiful human face staring back with wide eyes, saw something moving in the mirror atop the dresser behind her, and—
And had to stare himself. Because the creature standing on the reflected bed was more wolf than dog, longer than Spike had been before, leaner, lankier, more muscular. The fur was the right shade of purple, he was strangely glad to see, but it was longer, fuller, thicker, his tail much bigger and bushier. Forcing his eyes away from the mirror image, he looked down at his forelegs, flexed his toes, watched his fur ripple with sinewy strength.
"Oh, my," Fluttershy more breathed than said from somewhere to his left.
From the same direction, Sunset chuckled. "Looks like mirror travel agrees with you, Spike."
"Trust the magic." He dropped back to sit on his haunches, his gaze moving back to Twilight's face, the only sure point he'd ever had in his life. "That's what the other Spike said," he told the still gaping Twilight. "You didn't and I didn't and now...now, it's..." He raised a paw, not a twinge of pain anywhere in his entire body. "If I'd only listened to you years ago—"
"No!" Twilight clutched her hands in front of her chest. "I was wrong to force this on you, and I—" Closing her eyes, she turned away. "I'll understand completely if you want to go live with Fluttershy now or something. I tricked you and lied to you, and I—"
Without even thinking, Spike sprang from the bed, scooped Twilight up in his forelegs, and stood upright on his hind paws, Twilight startled and staring and perfect in his arms. "And I love you." He bent to touch his pointed black nose to her pert little purple one. "I may have bellowed that at the top of my lungs a few days ago, but I want to make sure you hear me say it again. 'Cause we were both wrong here, so how about I forgive you, and you forgive me?"
"Oh, Spike." Her eyes half-closed now behind her glasses, Twilight relaxed against his chest, brought one hand up to caress the fur at his cheek—
And a clearing of throat across the room snapped his attention to a wide-eyed Fluttershy and a grinning Sunset. "Okay!" Sunset said, crooking a thumb over her shoulder. "Might be it's time for me and Shy to hit the road."
"What?" Fluttershy looked back and forth between Spike and Sunset. "But...I have so many questions about how this could help my older canine patients!"
"Oh." Again, Spike moved more quickly than he had in years, setting Twilight back onto her feet and dropping to all fours, though even sitting down like this, he couldn't help noticing, his head was about even with her waist. "The other Spike didn't think that mirror travel would do anything to a regular dog." He had to swallow. "It's like...like the magic was just getting my outsides to catch up with my insides." When he shrugged, he realized that he could move his front legs in ways a lot more like human arms than should've been possible.
Definitely not a dog anymore, but certainly not a dragon, either. Still, every inch felt like him: for the first time in years, really...
"You guys don't need to go!" Twilight was saying, Spike gladder than ever that he could see the adorable colors her face was turning. "In fact, Sunset, you need to report the outcome to Princess Twilight! I know she'll be on pins and needles waiting to hear, and we'll need to set up some more comprehensive experiments to see how the mirror affects non-sapient lifeforms! Also, Fluttershy, could you maybe give Spike a quick exam here? We'll come in to the clinic for something more comprehensive as soon as you've got an available appointment, but—" She glanced down, and Spike's breath hitched at the sweet relief shining in her eyes. "For all indications so far, I'd have to say that he's definitely in good health."
Both Fluttershy and Sunset giggled, but Spike lost track of everything else in all possible worlds when Twilight's hand gently began scratching his ear.
"And the girls!" Twilight's hand pulled away, Spike almost falling sideways into her leg, he'd been leaning so deeply into the glorious sensation of her fingers. "We need to call them, tell them Spike's okay, invite them to the biggest picnic party we've ever had!"
Spike straightened to see her looking back down at him. "If you'd be okay with that?" she asked.
He had to leap onto all fours so his new tail could fully wag. "Right now, I'd be okay with pretty much anything." And while he didn't know exactly what he was other than Spike, he did know in a way that he'd never known anything before that he was going to be at Twilight's side for as many decades as she had coming to her. "And bring a tennis ball!" he called out, practically galloping for the bedroom door. "We've got some 'fetch' to play!"