Memorial pets are pets based on real life pets that Have passed away. They are not breedable, are not battleable, in fact they dont actually do anything- they are just here for remembrance....they also tell you uplifting things if you set them as your Active pet and will also find you items...just like all the others.
There you need to buy the specific pets items.
Then you use that item on any pet you have... There is no changing them back.