First you need create a new outfit animation... i recommend use _
OpenTibia - Assets Editor (https://otland.net/threads/assets-editor.273557/)
(Is like object builder but for Client 12x)
then it requires creating a new monster in the server files.. (there defines the characteristics of the bestary)
this is not enough for you to see the image in the client if you do these first 2 steps you will see only white in the client and it will not display a new monster..
Now, after doing the above with this application, edit the staticData... inside the Assets folder to include it so that the client sees the image of the monster.