Arwen is J.R.R. Tolkien's Elvish princess from The Lord of the Rings — half-Elven, immortal by birth, ultimately choosing mortality for love. That's an enormous amount of narrative freight for a pet name, and owners who choose it are almost always doing so knowingly. The name sounds beautiful and reads as literary without being obscure, which is a narrower target than it appears.
Pop-Culture Lineage
Liv Tyler's portrayal in Peter Jackson's film trilogy gave Arwen both visual and sonic familiarity across a generation. For pet owners who came of age with those films in the early 2000s, naming a female dog or cat Arwen is a direct personal reference point. It competes in the same fantasy-fandom naming space as Frodo, Gandalf, and Legolas, but skews more graceful than heroic.
Breed Preference and Sound Fit
Arwen's flowing two syllables — AR-wen — suit flowing-coated dogs. Afghan Hounds, Irish Setters, and long-haired cats receive it naturally. The name sounds ethereal enough to fit the aesthetic of a notably graceful animal. It's harder to picture on a stocky, rollicking breed, though nothing prevents it.
The Counter-Reading: Fully Fandom-Coded
Arwen immediately identifies the owner as a Tolkien admirer. That's a feature for some and a mild point of self-consciousness for others — especially if the owner has since moved on from a Lord of the Rings phase. The name wears its origin openly and doesn't pretend otherwise.