Okey so some mount's dont have any "dispaly id " like the Flying Reindeer or Merciless Nether Drake but u can fix them or make them look like a brand new mount !
First go to Wowhead: Epic like your faction leader. and search for the mount spell that dosent have any display id ( Merciless Nether Drake ) when u find it click on the spell thing
and u find some thing like this
the display id is how the mount will look so just put what the want the mount to look like and you are done
now u can just learn the spell for Merciless Nether Drake (.learn 44744) and u got your new mount =)
if u want the red nether drak here it is Filebeam - Free Fast File Hosting
Just Execute it to your database