you are asking ALOT here, Let me explain.
To tame ALL monsters you need the following
All mobs placed into the pet_db.txt table
You need to create a taming item for EVERY one, this means thousands of new items place into the item_db2 sql table
You need sprite files for every tame item
You need to edit the description files and add your tame item
this could takes weeks to do unless you work nonstop untill it is finished, Now i haent looked to see if there is a wiki entry to make pets, if not i will write one this week and post it for you so you can do it.
Edit:
There is a Custom Pet entry, it is located Here: http://rathena.org/wiki/Custom_Pets