Open the creature formation window by pressing ALT+Y and place the card for the tamed pet you want to summon into the window. You can now cast summon creature to bring it out. If you become one of the summoner classes you will eventualy be able to place 5 pets in your formation window. Once you have more than 1 pet in the formation you will need to select the pet you want to summon in the creature control window that should automatically open in the lower right corner.