Okay i know a lot of people don't know how to do this (I didnt). I just figured it out from messing around with the tables in the database. So here it goes. Step 1.Connect to your database with your database tool (SQLyog, Navicat, heidi etc..)
Step 2.Click on the Creature_names table and go to your table editor.
Step 3.Make a new row. For entry id, use something thats not already taken by an other npc (this will be .npc spawn <id you chose>)
Subname is the text you see under the creature name in <brackets>
Set flags1 to 0
Type = 0
Family = 0
Rank = 0
unk4 = 0
SpellDataID = 0
For display id, go to Wowhead: Preventing wipes since 2006. and type in the name of the npc that you want your custom mob to look like. Once you've found the NPC you like, look at the number at the end of the url it should be something like 5858 (Greater Lava Spider). Next go to your database tool and look in creature_names for the entry id that you found on wowhead. Once you've found it, look at the display id that it has, and put that same display id (in the Greater Lava Spider's Case 7510) into your custom mob's display id column.
unk2 = 1
unk3 = 1
Civillian = 0
Leader = 0
Whew! where done with creature_names
Step 4.Go to the Creature_proto table in your database tool.
Step 5.For entry ID, put the same number you put in for the entry id in creature_names(If you dont do this, it wont work)
Level = Your desired Level
Faction = 14
Health = Your desired Health Ammount
Mana = Your desired Mana Ammount
Scale = Your desired mob size
npcflags = 0
attackTime = Your desired attack time
mindamage = Your desired Minimum Damage
maxdamage = Your desired Maximum Damage
rangedattacktime = Your desired attack time
rangedmindamage = Your desired minimum ranged damage
rangedmaxdamage= Your desired maximum ranged damage
**NOTE** I havent found the ranged attack speed/dmg to affect the damage of the mob, but I would put it in just in case.
Mountdisplayid = 0
All of the itemslot/displayid/info = 0
Respawn time = Your desired respawn time (miliseconds i think) Default is 360000
All resistances0-6 = Your desired mob resistances
Combat reach = 0
Bounding radius = 0
Auras = Leave it blank
Boss = 0
Money = Your desired ammount of money dropped by the mob
Wewt we're done with creature_proto
Step 6.Now we will add the mob's loot.
Index = Use 9999999 To be safe (Yes, that many 9's )
Entry = Your Custom Mob's entry id (999999)
Item ID = Entry id for the item you want your mob to drop. (For mulitple drops, use the same creature id but a different index number)
Percentchance = Your desired item drop rate.
mincount = Your desired minimum ammount of how many times the item drops per kill.
maxcount = Your desired maximum ammount of how many times the item drops per kill.