on mob_db.txt add the item ID and drop chance you want, like this:
1213,HIGH_ORC,High Orc,High Orc,81,4193,1,1099,1158,1,389,439,101,45,75,16,40,31,69,20,10,12,2,7,43,0x3695,150,1500,500,1000,0,0,0,0,0,0,0,7002,2500,1304,10,999,90,931,7500,912,1300,756,196,508,900,607,100,0,0,4322,1
note: follow the struct because one monster have max number of item drop