Sulley is James P. Sullivan from Pixar's Monsters, Inc. — the enormous, blue-and-purple-spotted monster with a terrifying roar and an extraordinarily gentle heart. At rank 2554 with 36 registry appearances, pets named Sulley are almost exclusively large, fluffy, slightly intimidating-looking dogs with the softest personalities imaginable.
The Pixar Connection
The 2001 film Monsters, Inc. gave Sulley — and the nickname Sully — a specific character template: big outside, tender inside. This is the defining quality of the name as a pet choice. A massive Newfoundland or Great Pyrenees who wants nothing more than to lay his enormous head in your lap is named Sulley in the most perfect possible way.
The Spelling Choice
Sulley with the -ey ending is the Pixar spelling; Sully (without the E) is the more common surname form, also associated with the 2016 Clint Eastwood film about Chesley Sullenberger. The pet name market splits between both spellings, with Sulley carrying more explicit Pixar DNA.
The Counter-Reading: Requires the Physical Match
Sulley works perfectly on a large, fluffy, gentle giant. On a small or sleek dog, it reads as ironic — which can be charming but shifts the name's meaning considerably. Make sure the animal will grow into the name's physical premise before committing.