Alright before I start of I want to tell you that you wont make actual custom mounts, but it's more of less editing existing ones.
First of:
Step 1(doh):
First you will need a mount from wich you can edit it's currently active model from.
I usually take mounts taken out of the game(For example: Example 1, Example 2), or mounts never added in-game (Example).
Step 2:
Once you have decided which mount you are gonna edit, go to Wowhead for example.
While there type in the name of your mount, then take the SPELL of the mount, not the actual mount itself.
Somewhere in the spell details it says: ''Apply Aura: Mounted (xxxxxx)''
From that you'll need the number, remember it, write it down, I dont care
Picture:
Step 3:
For this step you will need the model that you want to mount.
For this preview I will use a log.
From this creature that you want to mount you will need the displayid, get this by logging into your private server, spawn your creature and type .npc info (depending on your emulator).
Remember this number aswell.
Step 4:
Open op your SQLeditor and open Creature_names:
Then in the Creature_names batch thingie search the number that you found in the spell details on wowhead.
In my example it will be: ''10179''.
When you find that number that matches ur mount npc, scroll to the right and find the fields: Maledisplay and Maledisplay2 and replace what is in there with the display ID you want it to be.
Step 5:
Confirm your settings in the SQLeditor that you are using.
Reload your creature_names table.
Learn the mount spell that we have just edited.
Have fun!