Sirius Black was naked. Most babies are when they're born, after all, except for Albus Percival Wulfric Brian Dumbledore who was born wearing a purple banana suit and go-go boots, but, you know. That's Dumbledore for you.
Anyway so Sirius Black was naked and screaming because he was a baby and that's what babies do when they first experience the world because the world is SCARY, you guys. Seriously, have you ever watched, like, the nature channel? I have. And I have learned that the world is friggin terrifying. Especially if you're a sluggish zebra.
Soooooooo Sirius was all AAAUAUUUUUGHJHJJJJ THE WOOORRRLLLDDDD and his parents were just like, ugh. Shut up. Because I don't know if you've heard but they were pretty terrible people. You know why?
THEY WEREN'T ACTUALLY HIS PARENTS! They stole him from a unicorn! Yes, Sirius Black's mother was really a unicorn. Neither of the Blacks knew who his father was but they didn't care, they just saw this unicorn with this baby future-ex-convict and they were like OMG FREE BABY! NOW THAT'S A GREAT DEAL! (They were always looking for good deals.)
And then they stole him. It was pretty easy, they just picked him up and walked away while the unicorn was busy posing for Lisa Frank stationery. When the poor unicorn mama got back to the stroller she'd left her newborn Sirius in, well she just cried and cried and cried and cried and cried.
So now he was naked and screaming in his Fake Mom's arms as his Fake Dad looked on and grumbled about unicorns eating all his prize begonias because his Fake Dad was a gardener extraordinaire.
"What should we name him?" asked Fake Mom.
"Begonias," muttered Fake Dad.
"Perfect!" said Fake Mom. "Hewo widdle Begonias!" she cooed at the baby, but suddenly Sirius's eyes narrowed and he pointed a tiny finger accusingly at her face.
"MY NAME IS SIRIUS," he announced in a voice so loud that it shook the entire Earth like a…um…like a milkshake?
Then he went back to burbling and gurbling and wurbling and murbling innocently as babies tend to do, and Fake Mom blinked at him in surprise.
"Sirius, then," she said faintly.
"Begonias," agreed her husband. He was so worried about his begonias that he forgot all the other words in the English language and never said anything else ever again.
Sirius smiled a wicked, impossible-for-a-newborn smile.
AND SO THE LEGEND BEGAN.