Millie ranks #112 with 947 entries and belongs to the family of -ie/-y diminutive female pet names that small-dog owners have favored for at least three generations. The name reads as warm, slightly old-fashioned, and unambiguously affectionate. It is what owners pick when they want a name that signals the relationship before it signals anything else.
A diminutive without a parent name
Millie originated as a nickname for Mildred, Millicent, and Camilla, but most owners no longer connect it to any parent name. It functions as a standalone now, the way Bobby and Eddie became standalones for an earlier generation. That detachment is what lets the name work for pets cleanly. Owners are not naming the dog Mildred and shortening it; they are naming the dog Millie because Millie sounds like the dog.
The name does well on small companion breeds where the affectionate register fits. Cavalier King Charles Spaniels, Bichons, smaller poodle mixes, and the warmer-toned terriers all carry Millie naturally. It is rare on working breeds and rare on cats, though the cat appearances are growing. The name fits cleanly into the broader small-companion register you can browse at pet-names.
Sound profile
Two syllables, stress on the front (MIL-ee), with the double-L glide that female pet names lean on heavily (Bella, Lilly, Molly all share the structure). The name is recall-acceptable but not commanding. The soft consonants and vowel-ending tail mean Millie carries best in close-quarters affectionate use, less well across a noisy park.
The royal pet effect
Queen Elizabeth II's corgis included a Millie, and the British royal pet vocabulary has had a quiet but real influence on American small-breed naming. Holly, Willow, Whisper, and Millie all appeared in the royal kennel records and all show up in our top rankings. That correlation does not prove influence, but the timing tracks.
One counter-reading: Millie has also climbed sharply on the SSA baby chart since the mid-2010s, and the same name overlap that affects human Millie is starting to register at dog parks. If saturation is a concern, the affectionate register is still wide open at names like Posie, Bunny, and Tilly.