They are mutations that (IMO) should've been culled, but in the same order as propensity to want balloon goldfish, are found by some to be novelties, and become marketable.
In any cichlid spawn there will be mutations that in nature would be eliminated by the laws of survival of the fittest, or if better suited to the environment, survive and take over.
When these mutations occur in aquarium spawns, and someone removes and saves these anomalies, and then breeds them together, we get the short bodies (with crammed together inner organs) screwed up mouth parts and forms (like BRPs) and the host of others we find more and more in our Pokemon induced Anime society.