I'm going to venture a guess on this one. I saw something today that seems like the same thing that you're describing. A pleco with a premanent crook in it's spine at the rear end of the dorsal fin. I'm going to guess that it's likely a pleco that's been returned to the store after outgrowing it's tank and developing the crook after spending too much time in a bent position. Just my uneducated guess.