by James Goblin » Sat Jan 17, 2015 7:46 pm
There should be a quest for each Imp (somewhere around very first levels, close to spawn point) Void (starts bit before or at lvl 10!?), Succy (starts at 20!?), Felhunter (30), Infernal (was it 50?) and finally Doomguard around 60th. These quests were removed later but if I remember correctly you had them throughout vanilla (and even much later).
PS some of these might be chains, so don´t expect pet as the very first reward.
PPS you also have quest for your Felsteed, mount (@ lvl 40 IIRC)