Mouse is the nickname-as-official-name scenario in its purest form. Seventy-three dogs are registered as Mouse, a name that almost certainly began as an informal tag — for a dog who is small, quick, or perhaps unexpectedly skittish — and got written down on the license application as-is. At rank 1,466, it's a genuine choice, not a clerical error.
The Irony Name and the Sincere Name
Mouse on a dog functions in two very different registers. On a tiny dog — a Chihuahua, a Italian Greyhound, a toy anything — Mouse is a sincere descriptor, affectionate and accurate. On a large dog, Mouse is pure irony, the same instinct that produces dogs named Tiny or Bear as joke inversions of the animal's actual size. Both versions show up in the registry.
Sound and Recall
Mouse has a sharp M opening and a drawn-out vowel that carries well across distance. It's distinct from most other pet names phonetically, which means a dog named Mouse is unlikely to respond to commands directed at dogs named Milo or Max. For owners who do a lot of off-leash time in crowded parks, that sonic uniqueness is a practical advantage.
The Nickname Problem
Mouse as a registered name means there's no obvious longer form to fall back on at the vet's office. It lives or dies as a single-word identity — which, for the right dog, is exactly the charm. If you want something that can expand into a full name when the situation calls for it, consider Moose as a phonetically adjacent alternative.