Newt really didn't mean to adopt a baby dragon.
Honestly, he didn't. He was in Norway to catch some graphorn smugglers and it wasn't his fault they had a dragon. He couldn't very well leave the poor thing; it was far too young to be on it's own, it was clearly malnourished, and it appeared to have injured one of its wings trying to escape. He found it in a cage at the back of the smugglers' base, an abandoned cabin in the mountains.
He had incapacitated the guards with a few quick stunning spells and intended to leave with the graphorns before anyone noticed, but then he heard a sound. A tiny screech, barely audible from his location. He followed that sound to a closed door. It was locked, but Pickett soon solved that problem. And in a shadowy corner between crates of supplies, was the dragon's cage.
Newt approached cautiously, slowly, crouching to make himself small and nonthreatening. "Shh, it's alright," he whispered to the creature. "I'm here to help you."
The beast watched him warily, but it let him come closer. He cast an unlocking charm. The door popped open and the dragon jumped back startled. A few sparks flew from its nostrils. Newt continued to speak to the creature, and eventually it came towards him, hesitantly placing one paw outside the cage.
Gradually, it came closer and eventually allowed Newt to touch it. "That's it, little one, you're safe now, I won't hurt you," he soothed. Once he got it into the case, it allowed him to bandage it's injured wing before falling asleep on Newt's bed.
Tina was going to be furious when she found out about this.
The dragon fit in quite well with the other residents of Newt's case. This was fortunate, as Newt had known from the beginning that the creature would probably never be able to return to the wild, since he had been so young when the smugglers captured him. Newt had named him Finn and built a habitat with pine trees and mountains and a sky that seemed to stretch on forever.
Dougal showed Finn around and introduced him to the other creatures. The nundu curled up to nap with him on sunny afternoons. The mother occamy helped him learn to fly after his wing was healed and seemed to have claimed him as one of her hatchlings. Finn was even happy to help Newt carry the feed buckets when he made his rounds.
Tina and Finn's first meeting did not go as well as Newt had hoped. She came down into the case one morning to drag Newt out for breakfast and nearly jumped out of her skin when she found him curled up with Finn, who was by this time larger than Newt and purring like a freight train with his wings stretched protectively over his human. Of course, once Newt introduced them properly, Tina and Finn got on just fine, though it took her a bit longer to forgive Newt for having gone into the smugglers' hideout by himself.