A Rogues Gallery Transplant is what happens when a villain is originally introduced as an enemy for one specific hero, but later becomes (through continuity creep) much more identified with another hero. While any shared universe may depict a hero fighting another hero's antagonist, usually they remain identified with the original. For example, Superman might occasionally clash with The Joker, but no one would ever say that he was anything other than Batman's villain. A Rogues Gallery Transplant is when the villain is now more identified as the adversary of a character he originally did not fight.
A good example would be the Kingpin from Marvel Comics. Originally, when he was introduced back in the early 1970s, the Kingpin was conceived as a Spider-Man villain. These days, especially after Frank Miller got his hands on the character, the Kingpin is associated with Daredevil to the point that when they finally got around to making a Daredevil movie, the Kingpin was the first choice of villains for the film.
This often occurs when an obscure character is used as a one-time antagonist for a new hero, but turns out to be much better suited to that new hero than the original one he was intended for, either thematically or in terms of powers. It can also happen when a writer creates a villain while writing one character, then moving on to another project and taking the villain with him.