First of all, we can't ADD new custom mob but REPLACE.

Mob sprites should put in this folder:
data\sprite\¸ó½ºÅÍ\

To replace, just look for any official mob you want to replace.

Example to replace Poring sprite with my custom Anita sprite:
Rename anita.spr and anita.act to poring.spr and poring.act and
put it in this folder ¸ó½ºÅÍ

That's all and below is a list of custom mobs ID that include in hexed client
that you can use.

ID	Display mame	Sprite name
----------------------------------------
1098	Green Jaguar	jaguar
1136	-		custom1
1137	-		custom2
1168	Scorpion king	sk
1237	Hollow poring	hollow_poring
1238	Dark poring	adarkporing
1239	Fire poring	fire_poring
1393	-		custom3
1394	-		custom4
1411	Pumpkinring	pumpkinring
1414	Mastering	master
1496	Ayame Man	ayame_man
1501	Giant Baphomet	giant_baphomet