"What is it?" Rysn demanded.
"Why we came here", Vstim said. "The thing we trade for, a treasure that very few know still exist. They were supposed to have died with Aimia, you see. I came here with all these goods in tow because Talik sent to me to say they had the corpse of one to trade. Kings pay fortunes for them."
He leaned down. "I have never seen one alive before. I was given the corpse I wanted in trade. This one has been given to you."
"By the Reshi?" Rysn asked, mind still clouded. She didn't know what to make of any of this.
"The Reshi could not command one of the larkin", Vstim said. "This was given to you by the island itself. Now drink your medicine and sleep."
Larkins must be really special if kings paid fortunes for them. I reckon they must do something special for the money. Even if they're dead. So clearly kings don't pay fortunes for them as pets.
Also species native to planets have the habit of spreading their habitats, even if they evolve in the meantime. So even if the larkin was endemic to Aimia, I would expect it to see somewhere else too, not just confined to that place and willing to die with it. The distance between Aimia and continent looks flyable. Moreover, just because they were said to have died with Aimia doesn't mean that's where they're from.
We also know that merchants do travel through the perpendicularity of Roshar and at least some must've gotten to the Drominad system.
And the larkin was given to Rysn by an island, a living one, which is too much of a coincidence for the Brandonverse.
If Aimia was always so isolated, maybe there's a gate on it to allow larkin trafficking.