"Smolt" is a real word in English. It means a juvenile fish.
In my headcanon, Megamind's species are called the Yi#velit, plural; an individual is called an Yi#vi. The # represents a gulping sound in the back of the throat; I put it in because I figured an alien language should have at least one sound that isn't in English.
When I was a real little smolt, I learned the kids' version of the story:
The Opuulu came up from the deep water.The Yi#velit came down from the sky.
They came together in the shallows.
That's why we call the shallows home.
It rhymes in the original.
This is the story I learned later, when my tutor figured I was ready for it.
The Opuu were native to Lup, our planet. They were deep ocean predators and scavengers, very tough and adaptable, with an instinct that would tell them what was food and what wasn't. The young had very flexible bodies, but when they reached puberty their skins would harden into armor. The young wild Opuu could survive being thrown to the surface, or into fresh water, but if they didn't get back to the deep ocean in a few days, they would adapt permanently and they could never go back because they couldn't survive the pressure anymore. Then when they grew up, the armor would make them too clumsy for shallow water life in the wild.
The Yi#velit were colonists. When they first arrived, they lived under clear domes. They altered their genome to turn their skins blue so they could go out in the sun, but almost all of them were still living on food species from their homeworld, which they grew inside the domes. The exception was a little group called, I guess it would translate indigenizers. They were trying to adapt to a diet of native foods.
Well, one day there was a hurricane that spun off tornadoes from itself, and one little Opuu was thrown up to the surface by the hurricane and then picked up by one of the tornadoes. That same tornado ripped open one of the Yi#velit domes and picked up a house with a family in it. It carried the house and the little smolt way inland and dropped them into a lake. The family inside the house were all killed except for a little girl, seven years old, named Reang Iworyeh. And this little girl and this little smolt found each other and they developed a symbiosis. The practical part of it was around shellfish. The Opuu would find them and the Yi#vi would open them. But they were also alone together in an area where neither species had ever been before, and they were both social animals who needed company, so they really bonded and learned from each other. They were the first pair.
Two years later, an Yi#velit exploration team came into the region and found a girl who was practically amphibious and a smolt who could talk, because she had learned how to shape her mouth and push water or air through it to make the sounds of Yi#velit speech. She was the first of the Opuulu, the talking Opuu. All of us are descended from her. The explorers brought her back to Yi#velit society, since it was way too late for her to go home to her own people. She and Iworyeh refused to be separated, so Iworyeh's surviving relatives adopted her into the Reang family as her sister, with Jilu as her personal name. They built her the first suit, voice controlled, so she could walk and do things on land.
The indigenizers glommed right onto Iworhey and Jilu. What they had been trying to do in a gradual, careful way, Iworyeh had done fast because she had to. They built this whole indigenization training program around the pair, who were happy to be the teachers in this program because they could live in the shallows the way they were used to. Yi#velit who'd been through the program would settle down on some fresh-water shore, or later on ocean shores, and villages would form.
Nobody knew how old Jilu was, but she reached puberty a few years after Iworyeh, about the time Iworyeh met the guy she ended up marrying. When Jilu hardened, she lost her abilty to talk, because she couldn't change the shape of her mouth to make the sounds anymore. So she got the first mod: a neural implant that connected her brain to a voice synthesizer, so she could still communicate and she could still control her suit.
Then when Iworyeh got pregnant, Jilu wanted to reproduce, too. So somebody went down and found a school of wild Opuu and harvested some milt. Don't ask me how they did that, but they brought it back and used it for artificial insemination. Jilu had twelve kids and every one of them bonded with a young Yi#vi in the indigenizer community. What's more, every one of them, when they got older, chose to have their hormones manipulated so they would never reach puberty, so they could continue to talk. This had the side effect of making them live as long as the Yi#velit, instead of dying at thirty or thirty-five, which was normal for the Opuu. When everybody realized that, they altered our genome so not reaching puberty would be automatic. The Opuulu basically gave up natural reproduction. We've been creations of science, made in labs, ever since.
Over the next few generations, the indigenizer community grew until there were about five thousand Yi#velit in it and most of them had Opuulu that they were bonded with. It was a lot of people, but still not very many compared to the whole Yi#velit population. Then there was an act of war. See, the Yi#velit competed with other spacefaring species. One of those competitors, the Equa, genetically engineered ten new diseases that wiped out the ten major Yi#velit food species. All the Yi#velit everywhere but on Lup just starved. On Lup, the indigenized Yi#velit became the leaders, since everybody else had to come to them to learn how to survive. Survival was a lot more likely for Yi#velit who had Opuulu that they were paired with, so all the Yi#velit parents wanted Opuulu to pair with their kids. (They tried it with adults and it just doesn't take. Something about neurological development.) It became normal for a family to reproduce in pairs, and after an generation it was normal for a family to start with four parents, two of each species. Every home had a tank at the center of it. Some of their technology was lost when the population crashed, and when it was built back up, it was built by and for both species together. In place of the old Yi#velit society that had been destroyed by the famine, there was a new society: the Paired Peoples.