Man, I no good at this but attempts were made! Anyway, headcannon time. I was inspired by DuneFilly ‘s headcannon about SK though mine is slightly different.
 
The yeti (his species) known as storm king originally started off as nameless, since his parents saw him as too weak to bother naming. He was forced to fend for himself as many young in his tribe did. While he was smaller than the others, he was also smarter and more resourceful, which led to him having more food than the others. After a while he realized how taking advantage of what others want was the key to manipulating others. He might have been weaker but his brains made it that he could collect food when others where howling at the wind.
 
As he grew older, he took more and more control, starting with the young and slowly consuming the tribe. He was dispised by the elders but no one could touch him as long as he provided food and ensured his tribe survived. It was only when he pushed a bit too far that he was eventually banished from his tribe. He had suggested that the tribe leave the frosted mountains of their homelands, to seek more than just survival. This was heresy and he was then banished, with only a few of his men following his lead.
 
He eventually found himself in the wastelands, where the underbelly of the world lived. While the weather was something he had to adapt to, the devious and cut-throat environment was his paradise. Slowly, he took over the town street by street, until he owned everything from the back alley trade-offs to the ‘livestock’ traders. He loved the power and wanted more,
 
His desire for magic really only came boat after meeting young Tempest, the lost filly with a broken horn. He’d never seen a pony before and while her magic was wild, it was powerful. It was power he wanted. Tales of how pony’s controlled weather and even the sun a moon made him hungry for that. If he could harness all that power, the world could be his. No one would dare look down on him. Except the pony princesses were strong and couldn’t be defeated that easily. He had vast swaths of land and technology the ponies could only dream of but their magic was a unknown variable he couldn’t over look. As such, he needed more power but that would require more manpower to do so.
 
He then returned to his homeland, smarter and more powerful than before. He found himself greatly disappointed by the fact that his homeland, his people, hadn’t changed. They still howled at the wind like dogs and acted like animals. It disgusted him. They cowered before him in his sky ship, snapping their fangs at his subordinates like beasts. So he told them of his travels and his accomplishments. The lands he’d conquered and the creatures he subjugated. This got him respect from the elders but disgusted him.
 
“Look at you cowards! Hiding in the snow like rabbits! You are mighty yeti and yet the world looks down on us. They call us monsters, savages! And looking at all you, basking in your ignorance of the world, I can’t blame them. But! That can change. If you follow me and become my loyal storm guard, I’ll show you the world and how easy it is to rule it.”
 
After that, he recruited tribe after tribe of Yeti’s, many through force but much more by sheer respect for his strength. Even Tempest, small as she was, gained the respect of the usually xenophobic Yeti’s. With them by his side, the kings hoard swept through the land, taking city after city. He took over as many ‘traders’ as he could, collecting more power and territory. Eventually, he noticed how folks would whisper about his descending on his enemy like a storm, so he took on the name, Storm King.