Hello all!
I learned that rare NPC's are very much worth killing while leveling, these mobs can give up to 3 times as much XP as a quest at an equal level +a couple of items!
I tried finding an add-on that could track lower level rare's without the need of nameplates/mouseover without any luck, however there is an add-on called _NPCScan where you can manually add NPC's of your choice (already including Outland and afterwards for achievements).
I have manually added every rare mob level 1-60 to my _NPCScan's lua file, and will upload the file so you can do the same
What you need:
_NPCScan
_NPCScan - Achievements - World of Warcraft Addons - Curse
Download this and put it in your addons folder.
The .lua file
https://www.dropbox.com/s/iac9yp5qhr...PCScan.lua.txt
This is the file that will add the rare NPC's to your database in-game.
This file needs to be replaced with the one in your WoW->WTF->Account->Account Name->Server->Character->SavedVariables->_NPCScan.lua (remember to remove the .txt at the end of the downloaded file)
(.. I am sure there is a much easier way to do this instead of manually doing it for every single character, however I'm pretty clueless around this sorta stuff, tried using my common sense with a mix of luck.)
In case this does not work the first time, simply open both files with text editor or a similar program and copy the entire document from the downloaded file into the current .lua in the folder.
You should now be ready, you can install and forget this add-on while leveling, whenever a rare NPC's is in range of you there will be a pop-up which works as a /target macro and the NPC will have a green triangle above its head
Important; when you find a rare NPC it will be remembered in your World of Warcraft->Cache folder, if you want to be able to track a mob which has already been discovered once, you will need to delete this folder (It doesn't seem to mess anything up regarding other addons don't worry)
Have fun!
Edit: To locate rare spawns by zone I recommend using wowhead, it has a feature that allows you to show rare spawns on the map of a zone.
Example: http://www.wowhead.com/zone=10#show:rare