To make the octopus hot dogs, cut each hot dog in half crosswise (the rounded ends of the hot dog will be the octopus heads). From the cut side, carefully split the hot dog halves lengthwise, leaving the last inch or so unsplit (i.e., don’t split the hot dog half all the way through). Split the half-dog once more to create 4 tentacles. You might end up with a severed octopus leg here or there, but don’t worry.
In a medium skillet, heat the oil over medium-high heat until shimmering. Add the hot dog octopi to the pan and cook, stirring frequently but gently, until they start to brown at the edges and the legs curl up, 3 to 4 minutes. Transfer the dogs to a plate while you make the sauce.
Add the onions to the hot pan and cook just until they become fragrant, less than a minute.
Add ¼ cup water, followed by the shoyu and brown sugar. Stir until the sugar is dissolved.
Continue cooking until the sauce starts to thicken, 3 to 4 minutes. Return the hot dogs to the pan, tossing them gently to coat in the sauce. Season with a pinch of pepper. Remove from the heat and serve as a pupu or over rice (if using), with any remaining sauce drizzled on top.
