Man, this is one of those things that feels like it's been around forever in paranormal romance circles, but when you dig into it, the concept of werewolves specifically living for a thousand years or more is a pretty modern mashup. Classic folklore werewolves weren't immortal; they were cursed humans, often with tragically short, brutal lives. The idea of an ancient, near-immortal lycanthrope seems to have crystallized from blending vampire lore—where antiquity equals power and romantic tragedy—with shifter mythology.
You can see early seeds in Anne Rice's vampire chronicles, where the ancient immortals brood over centuries. That 'ancient and powerful' archetype got grafted onto werewolves in the urban fantasy boom of the 90s and 2000s. Patricia Briggs' 'Mercy Thompson' series has werewolves who can live for centuries, their long lives creating complex pack politics and deep, historical grudges. Kelley Armstrong's 'Otherworld' series also plays with long-lived werewolves, exploring the psychological weight of centuries.
But the real explosion of the 'millennium werewolf' as a distinct trope happened in paranormal romance and romantasy, where that immense lifespan creates a perfect storm for romance: centuries of loneliness, a timeless alpha presence, and the tension of a mortal human entering their eternal world. It's less about horror and more about epic, timeless love stories and the burden of memory, which I think is why it resonates so hard now.