Guardian pet

Guardian Pets are one of three types of pets that can be summoned/created.

They will enter into combat on their summoner's behalf, much as a combat pet will, but cannot be directly controlled. They react as a combat pet set on Aggressive and Follow.
They are generally summoned on a temporary basis, unlike combat pets or Small Pets.

Most guardian pets "share a cooldown" with other guardian pets of 1 minute.

Most non-consumable items (of all varieties) that generate guardian pets have a longer cooldown period than 1 minute. For these items, it is more descriptive to say that they force a global "guardian pet item" cooldown of 1 minute.

This means that while you may not summon a second guardian within that 1 minute, you may use each of your pet summoning items in turn. For instance, once you've waited a minute after using your [Mithril Mechanical Dragonling], you can then use your [Mechanical Dragonling] trinket.

Engineering Guardians

Many guardian pets are created by and for engineers.

Some engineered guardian pets are consumable items, and can be used only once per item:

Other engineered guardian pets are reuseable. These generally take the form of trinkets. While these pet may be killed by damage, you simply wait for their cooldown and you can use them again. Their cooldowns range from 1/2 hour to one hour between uses.