Haesu was originally a human, we saw his child form in that one panel covered in black blotches, which is probably the contagious skin disease he was talking about, so my guess is that he either drowned in the pond, or his community at the time drowned him to get rid of the disease because I also remember him mentioning—in those early chapters—that he was not well liked, or maybe the community sacrificed him or something, hence the tree with ropes
Haesu was originally a human, we saw his child form in that one panel covered in black blotches, which is probably the contagious skin disease he was talking about, so my guess is that he either drowned in the pond, or his community at the time drowned him to get rid of the disease because I also remember him mentioning—in those early chapters—that he was not well liked, or maybe the community sacrificed him or something, hence the tree with ropes