Pet Interface

A relatively new interface for video games in which you are given a sidekick, usually a pet, that acts as a guide/virtual pet of sorts in the world. You can play with it, it will alert you of enemies, it will dig up hidden items, sometimes you can ride it, it usually can't die except in Plotline Death.

Sort of a low key Exposition Fairy crossed with Nintendogs.

Of course, since writers have known for decades that killing the main character's adorable pet is the cheapest and easiest way to get to an audience emotionally, you might want to read a FAQ and spoil yourself before you get attached to any of these. This can of course lead to So Long and Thanks For All the Gear if you've leveled up the pet.

Action Adventure

 * Agro from Shadow of the Colossus, to some extent.
 * Greatly expanded in the follow-up, The Last Guardian which is the story of a boy and his giant pet colossus griffin-thing.

First-Person Shooter

 * Possibly the Guidebot introduced in Descent 2.
 * He becomes more of an interface element in Descent 3

Hack and Slash

 * One of the most attractive features of the PC game Fate is the fact that your avatar is given a pet sidekick. You can name the animal, decide if you want a cat or a dog, and choose from a handful of different color variations. Although not invincible, the pet will fight for you and takes far less damage. Farther into the game, you can feed your pet different kinds of fish to turn it into a giant spider, a unicorn, or a number of other things, each with their own attributes.

MMORPGs

 * Several classes get pets in World of Warcraft. Elementals can be summoned by Shamans (earth, fire) and Mages (water), Priests get a shadowfiend, Death Knights can raise a ghoul from a corpse (even that of a dead player!), and Warlocks can summon a variety of demons, from the lowly Imp to the massive Infernal. But none are as pet-focused as Hunters, who not only can tame almost any Beast-type enemy in the world, they get to name the pet and give it its own minor talent tree. In all of the above cases, the pet gets a small bar of icons above the player's Action Bar, for the player to give commands with.
 * Guild Wars, likewise, allows player characters to train pets using the Ranger class' Charm Animal Skill (which any character can gain access to by switching their secondary profession to Ranger). Actually having the pet show up requires Charm Animal to be equipped, and other skills do things like healing, buffing, and reviving it. A character's pet can be named and if charmed below level 20 will level up and gain different, mutually exclusive traits as it levels (for example, pets that took dramatically more damage than they dished out will gain a permanent bonus to hit points at the expense of attack damage). Characters can unload a pet on an NPC "trainer," at which point the pet is Lost Forever, but opens up the slot for a new pet. Pets will hide in Hammerspace when in towns or when Charm Animal is not equipped.
 * Wizard 101 has pets that are raised to compete with other players in races and provide the occasional stat boost or spell. There are over a hundred different pets that can be obtained from various ways ranging from boss drops, quest rewards, to free with the purchase of a gaming card.
 * Star Wars: The Old Republic has this as one of it's many unique features in that your "pets" are fully fleshed out companions with unique combat roles and reactions to certain situations. They can even be sent on missions or given crafting orders to keep them busy while you're out on, say, Tatooine killing sand people. Just make sure you leave one with you to help in combat, because the game's balanced in a way that makes solo combat without a companion somewhere between diffcult and impossible.

Platform Game

 * Tricky the "Triceratops" from Star Fox Adventures.
 * The ENTIRE Spyro the Dragon series. Sparx the Dragonfly is your health meter (Gold -> Blue -> Green -> Gone, later updated to include an additional level of health, either a glowing gold or a warning red), and in later games can bust nearby gem cases, collect nearby gems, and even has his own levels.

Puzzle Game

 * In the Professor Layton games, you're given a pet that will point out hint coins for you and unlock a set of bonus puzzles once you've completed its particular minigame. In Curious Village, it was a robot dog you had to assemble; in Diabolical Box, it was a hamster you had to exercise; in Unwound Future, it was a parrot you helped make deliveries (this was changed from the Japanese version, where you taught the parrot different words instead). The fish you get in Last Specter still comes with a minigame, but it will find hint coins for you even before you complete it.

Role-Playing Game

 * The dog from Fable II, who
 * Which is similar to the Animal system in the Black and White games.
 * Not quite
 * The "dog after me own heart" in The Bard's Tale.
 * The game plays with parts of the trope:
 * Nall mostly provides snark in Lunar: Silver Star Story, but he's also the menu cursor and your Bag of Sharing.
 * Baby from the PlayStation RPG Guardian's Crusade, which revolves around this trope.
 * The transforming dog in Secret of Evermore.
 * Dogmeat in Fallout 3 is shaping up to fit this trope.
 * Specifically, you can order him to retrieve items for you in addition to siccing him on your enemies. There are also options to praise and scold him, but those are just there for flavor.
 * Skye from Grandia II is a bird that acts as a father figure and a bad mode of transport to the main character.
 * Final Fantasy VIII had one of the main characters have a pet dog which could help attack in battles, such as shoving it onto a crossbow and firing it at enemies. He could also rez KOed allies (and make the party invincible) and search for items on the battlefield.
 * Which is based on the "Animals" ability from Final Fantasy V, where a character could summon random animals to attack enemies, heal you, block attacks, etc.
 * Shadow's dog would occasionally enter battle for a free attack in Final Fantasy VI.
 * The Mabari war hound in Dragon Age (later named "Barkspawn" in DLC). Another Mabari shows up in Dragon Age II, though more as a Shout-Out to the first game (it's treated as a summoned creature, while the original dog was a full companion character).
 * In Skies of Arcadia, Fina has Cupil, a living weapon of sorts which acts as her pet and her only childhood friend.
 * Torchlight combines this with Bag of Holding; your pet acts as a secondary inventory as well as a combat helper, and can even go back to town to sell your excess loot for you. Somewhat unnecessary given how common town portal scrolls are, and you get a much lower sale price for your spoils, but nice for those times when you just can't be distracted.
 * Nethack - While not an integrated system, a surprising amount of Nethack's gameplay can be usefully applied to pets—or rather, tame monsters (which can include humanoids).
 * Cats, dogs, and horses can be tamed with appropriate food. Magical means permit other monster types to be tamed.
 * Carrying pet treats in open inventory (tripe for dogs/cats, pears/apples for horses) means your pet will stay much closer to you.
 * Tossing your pet a treat reinforces its recent behavior. You can train your pet to steal from shops!
 * A leash physically keeps your pet close to you, even when going up/down stairs or dropping through holes. (You can use multiple leashes, too.)
 * A tin whistle will kind of call your pet to you. A magic whistle will teleport your pet(s) to your side if they're anywhere on the level.
 * #chatting to your pet can tell you about their condition.
 * Pets are reluctant to step on a square with a cursed item on it. (unless there's food there they want.) This can identify cursed items. It can also be used to control a tame animal's movement.
 * You can 'a'pply a bullwhip at a humanoid pet to take their weapon away. (For replacement with a better weapon.) You can also prepare and leave weapons, armor, and tools for your humanoid pets to pick up and use.
 * Wand of probing tells what a monster (tame or otherwise) is carrying.
 * A stethoscope tells an animals basic hitpoints and it's speed.
 * Spells of healing and extra healing can be used to heal pets.
 * Breaking a potion of regular, extra, or full healing can heal pets.
 * Saddles permit horses, dragons, and some other monsters to be ridden.

Simulation Game

 * Harvest Moon has featured a dog and a horse in virtually every incarnation. Sometimes you also got a cat (or had to chose between the dog or cat).

Stealth Based Game

 * Metal Gear Solid 4: Guns of the Patriots provides the tiny, cute, adorably flaily Metal Gear Mk. II (and III). You can use it to do reconnaissance, fetch ammo for you, and extend a prod from it to electrocute Mooks. Find a dog that can do that.

Survival Horror

 * Brown from Rule of Rose.
 * Hewie from Haunting Ground.

Wide Open Sandbox

 * The dolphins in the Endless Ocean system.
 * The adorable Jeanne from No More Heroes. She gets fat in the second one, and if you get her to a low enough weight, Travis gets inspired and gains a new move