It is called "conjugation". I think they should extend the times they have for it though, for it really pisses me off when my best goes away in 30 seconds. However, not everyone is a mage and you can have assassins follow you (assuming you finished the Dark Brotherhood) and the same with mages for the mage's guild. I will go with for some classes.
I actually was thinking about it, and i realized that it could be a new skill. Te higher you petskill is, the more pets you can have at once, and the more they will listen to you. a Journeyman+ would also be able to give their pet's special armour and weapons.
Conjuration is for a short time because it summons a very powerfull being, rather than a less powerfull being for an indefinate ammount of time.
Edit: Grammar