Ball of Pet Summoning
Balls of Pet Summoning are dropped as uncommon loot by Solen Queens. Once "linked" to a pet, they can be used to summon that creature to the owner's location - even if the pet is stored in the stable. Although they retain their "linked" status when passed to another character, only the pet's actual owner may use the ball for summoning purposes. A different character may only remove the link and then create a new one with a pet of their own.
Many tamers make use of a different trick in order to "summon" their pets to difficult areas: Logging out and then back in will drag an unstabled pet from anywhere in the game world to the owner's location (so long as the owner is not dead).
However, sometimes (very rarely) a pet will become "glitched" (so that it cannot be targetted and will not log out with it's owner). In these cases only a pet ball can recover it; it is recommended to link all your valuable pets to pet balls as insurance against these scenarios.