what are they?

When a species undergo a revamp, sometimes certain traits are moved to different trait slots. This will cause them to no longer be able to exist with some other traits it previously was able to. When a pet is affected as such, the admin will "reset" the pet's traits to nothing, and the player can reassign the traits, based on the amount of trait points the pet had previously. This is called regeneration, and pets who are applicable for this are preventable, or regens.

It is kind of similar to creating a starter pet, just that the amount of traits you're able to assign, is based on the amount of traits the pet had previously.

The most recent species that underwent this is the lunemaras, where all the lunes that had glowing dust, swirling snow, amongst other traits, were eligible for regen.

You can read more about it in this thread: https://www.sylestia.com/forums/?thread=49886