Heat a large skillet over medium heat; prepare with cooking spray.
Stir pumpkin, soy milk, corn, eggs, and onion together in a bowl; add rice flour, baking, powder, and cinnamon and stir until well blended.
Drop mixture by desired portions onto preheated pan. Cook until lightly golden on the bottom, about 3 minutes. Flip cakes and continue cooking until the bottom is lightly golden, about 3 minutes more. Repeat cooking in batches until batter is completely used.