In RME go to File > Import > Import Monsters/NPCS , then navigate to the folder where .xml of your creature is and import it.
Then go to Creature Pallete > Others and it should be there.
The visibility of the monster in RME on map depends if the creature has a new [custom] sprite looktype or an already existing one. Sometimes, these creatures will be seen as: mage outfit (all white), or even not visible at all.
In order to fix this [for custom sprites], use OTItemEditor program to compile a new items.otb file for the server, then go to the folder where you installed RME--> Data--> 8.XX (version of tibia required for the map you are using) --> and replace the existing items.otb with yours. (make sure you create backup of old .otb, just in case.)