The seed you show, looks more or less like cottonwood. (which is not particularly good as a bonsai subject) but the seedling that popped up is definitely not a cottonwood. However, I would not expect cottonwood to sprout without winter cold stratification. So if you planted the seed in August. Leave the pot outside for the winter.. In spring there is a fair chance cottonwood seedlings will sprout. If you do not want to put the seed outside, put the pot with the seeds in it in your refrigerator for 3 months. Seed of most north temperate trees need a 2 to 3 month cold winter period before sprouting will happen. This is called stratification. You can leave the pots with seeds outdoors for the winter or stratify your seed in the refrigerator.