Abandoned pets, once bound by soul contracts to humans, but later released from their contracts.
Such a situation with abandoned pets can be both common and uncommon in the world of soul pets.
It is common because as human strength improves, some earlier soul pets become unable to keep up with their masters due to factors like aptitude and level. Once their combat effectiveness is greatly diminished, masters may discard their soul pets, typically by releasing them from their soul contracts.
It is uncommon because whether being released or when a soul pet dies, the soul needs a year to recover. During this year, the space reserved for abandoned pets cannot accommodate new pets, and only after a year can new contracts be signed for that soul.