Topsy and Mopsy entered Mopatop's Shop in the episode "Swapsies". They say that they love to do the swapsies, they do it so often that they don't really know that they do it. They have a little rime that they do each time "Ixie pixie poxy. Watch us do a swapsie". They had to do the swapsies a few times in the shop, since it was the only way they could get what they wanted.
The Mopsy puppet appears as the Jack Rabbit in Kermit's Swamp Years.