He came to the Apples when he was a filly, just after his parents died. He had walked to Ponyville from the outskirts of Baltimare when he was no older than the Applebloom Twilight had met, and when he found them, he was half-starved and crazed from exhaustion. Never one to turn away such a pitiful creature, they'd taken him in, where he collapsed and slept for nearly two days straight.
Upon waking, Prairie begged Granny Smith to give him a job; He was terrified of going into the or-foal-age. His older sister was already in one, and he had nearly joined her in there; The word she said to him after she was caught was simply, "Run." He followed her wishes without looking back. He never told them about his sister, however. Only that he didn't have any family or a place to stay, and he didn't want to go to the orfoalage.
So Granny Smith let him stay, and while he was terrible at bucking apples, his unicorn magic of basic levitation proved helpful in making cider, transporting apples, and various odd jobs. Applejack even gave him her other hat, a stetson just like her own, only black. She considered him a member of the family, and her little brother, as did Big Mac. Though Prairie would never be as strong as an earth pony, he still grew up to be a strapping young colt, and he did his work faithfully and as steadily as anypony else. Still, everypony on the farm knew apples wasn't his passion.
Applejack and Big Macintosh both already had their cutie marks by that point, but he was nearly a young stallion and still had no mark of his own. He knew, somewhere in his heart, what his special talent was. But in trying to test it out, he unwittingly changed the course of all of their lives.
One day, while accompanying Big Mac while he bucked apples, he gathered the courage to ask the strong, quiet stallion something he'd wanted to for quite a while.
"Uhmm... Big Macintosh?"
"Eeyup?"
After a few moments of opening his mouth to speak and then closing it again, and clearing his throat comically loudly, he blurted out,
"Can I draw you naked?"
"WHAT IN TARNATION?" A young Applejack suddenly burst through the apple trees before Big Mac could say a word.
"Applejack!" Prairie Fire's eyes grew wide and he tensed up, as if expecting to be attacked. Suddenly, his expression turned quizzical. "How did you get here so fast?"
"Now, Applej-" Big Mac started, but Applejack was having none of it.
"What in the hay're you saying, Prairie Fire? That's mah brother! What're you, a coltcuddler?"
Coltcuddler, coltcuddler! Coltcuddler, coltcuddler!
Prairie stepped back, his eyes suddenly too blurry to see the two ponies, and he shut them hard, hoping that indeed, he would never have to see them again. Her words rang in his head like a hideous bell. He turned and raced through the trees; Even blind, he hardly even stumbled. He knew that apple orchard nearly as perfectly as they did, and could navigate it almost as well.
Run run run run run run run, always running.
"Prairie Fire! Prairie!" Hollered Big Macintosh, but Prairie was too scared and hurt to think clearly and go back.
Soon, he was in his room, which had stopped being the 'guest room' almost a year ago, when they stopped thinking of him as a guest. He levitated his few belongings, as well as his saddlebags of all the earnings he had made on the farm. He raced down the steps, belongings in tow, nearly knocking over Granny Smith. She called out to him too, but he folded his ears down, trying to block out her voice, pretending he didn't hear her. Pretending he didn't know her, or that she didn't exist.
After he was out that door, he had seen them for the last time; At least, until Pinkie's party. He had artfully blocked out his memories of staying at Sweet Apple Acres. Loneliness was his only salve for a broken heart.
---
"So," He sighed, defeated. "You're still mad, huh?"
Applejack's teeth bared, and that fear spread through him again- She was going to strike.
"Yer darn tootin I'm still mad! How could ya do that ta us?"
As he flinched back, more memories flooded him; Where he had gone after he left the Apple family he had tried so hard to forget.