In the Grimm story where the fishes have a swimming contest in order to decide who is to become king, one of the contenders is the plaice (Pleuronectes platessa), who is not known for its impressive speed. It is of course crushingly defeated by the herring (and almost everyone else), and as the fishes cry out the news the plaice angrily screams, “who is first? Who is first?” The other fishes reply, “the herring! The herring!” The plaice shrieks, “The naked herring? The naked herring?” As a punishment for this jealous behaviour, the plaice has its mouth on one side from then on.
Explanatory stories like these are common of course, but this one appears to miss the point, as flatfishes don’t have an asymmetrical mouth at all. It is the eyes that are on one side (which side differs between flatfish species; the plaice has its eyes on the right side). They are not born this way though. A flatfish begins its life as a symmetrical being that swims the way fishes normally do, with the belly pointing downwards. As they mature, they start tilting sideways until they end up on one side. During this period of development, one of the eyes migrates to the other side of the head, which gives the adult its awkward-looking asymmetrical face.
Flatfish evolution seems to have followed a similar path, as fossils have been found of adult flatfishes (such as Amphistium) with a displaced eye that has not quite crossed over to the other side of the face yet. However, whether this entire evolutionary trend was driven by jealousy, I do not know.