The Necromancer can control the pet's appearance to some degree by right-clicking on a summoned pet and selecting Change Appearance. The appearance of these pets differs between types (fighter/tank pet, scout pet, mage pet), and also by the level of the pet-summoning spell, and by the quality of the spell (Apprentice, Journeyman, Adept, Expert, Master, Grandmaster, Ancient). Necromancers have a variety of combat pets they can summon, depending on level.