(a terminarch is another word for an Endling, or the last of a species)
Reason: I took this in another direction. The Vet pupper has the same color scheme and similar body structure as the Wasteland Wraith. Therefore, it seems safe to assume that the pupper is related to it somehow.
One definition of wraith is “the exact likeness of a living person seen usually just before death as an apparition”. Combined with the Wasteland Wraith’s effect, which is to wipe out everything, from that alone we can infer that something terrible happened in what we now simply call the “wasteland.” That event (which in my head-cannon is the Cataclysmic Fault) destroyed everything and left only death and ghosts.
Somehow the doggo survived, but his entire family and species was wiped out. As the sole survivor in a harsh and lifeless landscape, the player character found the wild doggo, hungry and alone. You, as a scion of the Vetruvian Imperium, (a descendant of a wealthy, aristocratic, or influential family), understand the bond of a family and the tragedy of loss. Having mercy on the poor creature, you saved him. You fed him, protected him, and gave him a family again. Now he’s bonded to you, and will give everything he has to protect you in return. He already lost everything once. He won’t let it happen again.