Many different Massively Multiplayer Online (MMO) games include or feature pets as an element within the world and story of the game. In many of them, these pets may provide simply an aesthetic touch to the world, giving the player's character a companion that can travel with them. Other games use pets as an element of combat within the game, allowing players to issue commands to them that are used to defeat enemies. There are also pet MMOs in which the focus is on training or raising these creatures, rather than on combat, and these games can be pet shop or zoo simulators.
Some of the most common types of pet MMOs are those in which pets are not necessarily the main focus of the game, but can be acquired or used in a number of different ways. Many of these games include items that can be found through various means that are referred to as "vanity pets." These are small creatures that can be summoned by a player and follow his or her character around. They do not interact with other creatures or players within these pet MMOs, however, and are merely there as "vanity" items to demonstrate accomplishments that have been achieved.
In some pet MMOs, the creatures that players can encounter and tame take on a much more interactive and important role within the game. Many games include "combat pets," which can be used by the player to take various actions within the game. Some of these pet MMOs are even designed with the creatures as integral parts of the combat experience for all players or certain types of characters within these games. As this genre has developed and grown, the role of combat pets in these games has changed. There is some controversy among players and developers regarding the effectiveness of creating characters that rely on virtual creatures rather than the players controlling them.
There are also some pet MMOs that are designed to feature animals as a main element of the game, without the use of combat or other systems. These games are commonly created as simulators or "sims" that mimic the experience of caring for animals. Pet shop simulators, for example, can allow a player to create a character and a shop that can be filled with animals as they are acquired, which need to be "fed" and "handled" appropriately. Zoo "sims" can also be quite popular as pet MMOs, allowing players to design and build a zoo which they then fill with various virtual creatures.