You have to do this...
Client Part.
1. jobname.lub
[jobtbl.JT_Addax] = "Addax"
- if it's the last one on the list, you should not put "," after Addax.
2. npcidentity.lub
JT_Addax = 3460
- if it's the last one on the list, you should not put "," after Addax. It is advisable to use the id of monsters that are not being used.
- The monster sprite is fine in that folder.
Server Part
An example for the mob_db.
3460,Addax,Chupacabras,Chupacabras,146,60935000,1,2010,2010,3,8375,12764,436,218,355,255,255,255,255,255,10,12,2,0,83,0x6283695,130,212,384,360,1005,617,5500,6223,2000,6223,2000,994,10000,2677,3000,2678,200,2679,200,1471,2000,1133,2000,12539,200,0,0,0,0,0,0
Only that is needed to implement a custom monster...