Pet

Pet is a common name for any summonable companion of a player. Pets autonomously and independently attack enemies (or otherwise assist in battle) while not being under the player's direct control. In fact, most pets act as monsters, except for they are fighting on player's side.

Pets are sometimes called minions, but that name is generally reserved for monsters accompanying a powerful enemy. Companions, summonables and summons are also common terms for pets.

Some pets are attackable by enemies and/or killable, while others are ignored by enemies for targeting purposes and have no Life bar. Most pets block enemy pathing, and therefore may lock their target in combat, taking hits on themselves while keeping their master safe. Unattackable pets, however, very rarely if ever block enemy path. In Diablo III, killable pets' Toughness depends on that of their master, and they take greatly reduced damage from area-of-effect attacks, as well as any attacks that are supposed to be avoided by the player.

All pets have their own AI, and usually attack the closest target they see. As enemies generally do so as well, melee pets are often used as a meat shield for their ranged-attacking master. In Diablo III, monsters tend to attack their master's current target, unless it is somehow out of their reach.

Pets tend to stay close to their master. If the latter ventures too far, pets can teleport to their current location, although immobile summonables lack this ability, and will stay where summoned until the location is exited, or until they are killed or their timer expires.

Followers and Hirelings are not necessarily pets, and should not be considered as such, though they share similarities with pets in the facts that they have their own AI, and also fall in battle if their master dies.

Necromancers, Druids and Witch Doctors are the three classes that have an opportunity to deal a major part of their damage through pets, although other classes may sometimes make a build focused on pets.

In Diablo III, pets of other players are displayed transparent and less visible.

Pets can be unsummoned at will (for example, if player does not want them to attract the attention of idle monsters). In Diablo II, it is done through the Unsummon skill, which any player gains if they have a skill that allows summoning pets. In Diablo III, unless pets are passively summoned, one can dismiss them by right-clicking their portrait (upper left corner of the screen) and selecting the appropriate line. Pets almost always despawn and die if their master is killed.

Skills that count as pets
(*) indicates that the summoned pet is not targetable at all, regardless of whether their master fights against the minions of Hell or other players.

Diablo

 * Guardian (*)
 * Guardian (*)

Amazon

 * Decoy
 * Valkyrie

Assassin

 * Shadow Warrior
 * Shadow Master
 * Charged Bolt Sentry (*)
 * Wake of Fire (*)
 * Lightning Sentry (*)
 * Wake of Inferno (*)
 * Death Sentry (*)

Druid

 * Raven (*)
 * Poison Creeper (*)
 * Oak Sage
 * Summon Spirit Wolf
 * Carrion Vine (*)
 * Heart of Wolverine
 * Summon Dire Wolf
 * Solar Creeper (*)
 * Spirit of Barbs
 * Summon Grizzly

Necromancer

 * Raise Skeleton
 * Clay Golem
 * Raise Skeletal Mage
 * Blood Golem
 * Iron Golem
 * Fire Golem
 * Revive

Paladin

 * Conversion

Diablo III
In Diablo III, pets can often be enhanced with items, like Enforcer, Tasker and Theo or Mask of Jeram. Unless specifically stated otherwise, their attacks will give no Life on Hit and will not trigger any 'on hit' special effects. In return, the Reflects Damage affix has no effect on pet attacks. Life per Kill bonus, however, is effective even if the enemy is killed by a pet.

Pets can trigger Area Damage (Patch 2.3.0, August 2015).

Only Tasker and Theo can increase the Attack Speed of Pets. Any other Attack Speed increase will increase the Pet's Damage instead. This makes Attack Speed critical for most Pet oriented builds.

Due to programming errors regarding AI, pets will sometimes just stand still in front of enemies without attacking, even if they get attacked, or will simply just walk with the player and not seek enemies at all. This mostly happens when the player has too many pets active and there are too many enemies on screen. Moving around or re-summoning sometimes fix this problem. In 2.1 this problem still happens, but less frequently compared to other patches.

Barbarian

 * Call of the Ancients

Crusader

 * Judgment (with Conversion rune)
 * Phalanx (with Bowmen and Bodyguard runes)
 * Falling Sword (with Rise Brothers rune)

Demon Hunter

 * Companion
 * Sentry (*)

Monk

 * Mystic Ally

Necromancer

 * Command Golem
 * Command Skeletons
 * Simulacrum
 * Simulacrum

Witch Doctor
Note: multiple passive skills and active skill runes allow summoning Zombie Dogs, even if the Witch Doctor does not have that skill)
 * Corpse Spiders (*)
 * Summon Zombie Dogs
 * Gargantuan
 * Fetish Army
 * Fetish Sycophants
 * Spirit Barrage (only the Phantasm Rune; despite being technically a Pet, it will NOT be empowered by Pet-enhancing effects with the exception of increased Damage from increasing Attack Speed)

Wizard

 * (with Fracture rune)
 * Mirror Image
 * (with Fracture rune)
 * Mirror Image

Non-combat pets
Non-combat pets can only be summoned from Wardrobe, and do not affect gameplay in any way except for collecting gold for player. Only one non-combat pet can be summoned at a time.

Diablo II
Only items with the actual ability to summon pets are listed; items that only enhance the existing summons are not included.
 * Beast Rune Word
 * Bloodmoon
 * Bramble Rune Word
 * Carrion Wind
 * Crescent Moon Rune Word
 * Death Rune Word
 * Delirium Rune Word
 * Dragon Rune Word
 * Dragonscale
 * Eternity Rune Word
 * Faith Rune Word
 * Harmony Rune Word
 * Heart of the Oak Rune Word
 * Hellfire Torch
 * Hexfire
 * Metalgrid
 * Nature's Peace
 * Oath Rune Word
 * Passion Rune Word
 * Peace Rune Word
 * Stone Rune Word
 * Tiamat's Rebuke
 * Wisp Projector
 * Wolfhowl

Diablo III
The list also includes the items and sets that indirectly increase the amount of pets summoned through skills.
 * Archmage's Vicalyke (indirectly)
 * Baleful Remnant
 * Belt of Transcendence
 * Bombardier's Rucksack (indirectly)
 * Bovine Bardiche
 * Corrupted Ashbringer
 * Embodiment of the Marauder Set (indirectly)
 * Genzaniku
 * Golden Gorget of Leoric
 * Haunt of Vaxo
 * Helltrapper
 * Inna's Mantra Set
 * Leoric's Gauntlets
 * Maximus
 * Murlocket
 * Puzzle Ring
 * Raiment of a Thousand Storms Set
 * Remorseless
 * Skycutter
 * Spirit of Arachyr Set (indirectly)
 * The Cloak of the Garwulf (indirectly)
 * The Crudest Boots (indirectly)
 * The Grin Reaper
 * The Raven's Wing
 * Serpent's Sparker (indirectly)
 * The Short Man's Finger (indirectly)
 * Unrelenting Phalanx (indirectly)
 * Visage of Giyua
 * The Short Man's Finger (indirectly)
 * Unrelenting Phalanx (indirectly)
 * Visage of Giyua