Pet Aquire Levels

Bolshka:

1 - 4 : Imp

10 - Voidwalker

20 - Succubus

30 - Felhunter

50 - Infernal -- Not really a pet as you have to enslave it after 5 minutes and keep trying to as the Enslave breaks. Enslave is subject to Diminishing Returns so lasts shorter periods when used on the same mob.

60 - Doomguard. Again, an Enslave but must be done immediately after Summoning. Suffers from DR as well.

Variable - Felguard. Requires you to spec Demonology. 41 point Talent.

The last three are taken from WoWWiki as mine isn't quite there yet.

Bolshka:

Your standard roster of pets aside, I just thought I would mention that having Warlocks around to enslave demons is a HUGE boon to running instances such as Mechanar or Arcatraz.

There are several spots in those instances with five-mob pulls. Enslaving just ONE demon makes it to where the rest of the party doesn't have to lift a FINGER to wipe 'em all out… and that's on HEROIC setting, folks.

Impressive stuff, really, because otherwise those pulls are a nightmare.