There is a new building at 51, 90 in Coldbreeze which contains two NPCs (perhaps more later), one NPC is a Summoner who will require various ingredients to help you summon a new pet from the Hell Plane.
To summon a pet you will need two items, a candle and a variable quantity of flesh, the candle is a special item that comes from the Demonology skill. The Demonology skill which you get when evolving into a Warlock at level 650 has 6 magic spells, these are for summoning candles. These spells are drop plan only (or Brain Expander/Scroll of Knowledge) this means they are rarer than ordinary spells and thus getting a new pet is cooler!
When you have the required candle and flesh turn them in to the NPC to get your new pet. The pets that come from the Demonology line are all pretty cool pets, and they have some pretty unique uses.
Hellish Imp, Casts hell magic spells.
Souleater, steals mana from a target and gives it to you.
Lifetaker, steals health from a target and gives it to you.
Hellwarden, has no offensive powers but has high health and has a high amount of weapon defense skills. With taunting turned on it can take a great deal of melee damage before dying, think of it as the meat shield you always wanted.
Bloodmaiden, steals stamina from a target and gives it to you.
Whelp, a mixed combat pet with various melee skills and average stats.
The candles can not be traded and the candle summoning spells require varying levels of Demonology skill to be able to cast the spell so that should ensure that you get a chance to learn how to use your pets.
The pets are quite varied and intended for different situations obviously the Souleater pet would not do so well versus a melee profession as it would have no mana.
Deomonology skill can be raised through casting the new candle spells or from taming Demon type monsters you find in the game.