Once you have obtained a Rotom Catalog, you can use it by selecting it from your Key Items pocket and then selecting the form you want to change Rotom into. Rotom will then change into the selected form, and its type and moves will change accordingly.
The different Rotom forms are as follows:
- Heat Rotom: Fire/Electric type with the move Overheat.
- Wash Rotom: Water/Electric type with the move Hydro Pump.
- Frost Rotom: Ice/Electric type with the move Blizzard.
- Fan Rotom: Flying/Electric type with the move Air Slash.
- Mow Rotom: Grass/Electric type with the move Leaf Blade.
In addition to the Rotom Catalogs, there is also a special item called the Rotom Bike. This item can be obtained by completing the Pokédex, and it allows you to change Rotom into a bike that you can ride around on.
To change Rotom into the bike form, you will need to use the Bike Key item. Once you have used the Bike Key, Rotom will change into the bike form, and you will be able to ride it around on the ground or on water.