Sullivan is an Irish surname meaning "dark-eyed" or "hawk-eyed," from Súileabháin, and it's made the surname-as-first-name migration so successfully that it now appears on both children and dogs with equal frequency. On a dog, Sullivan has a specific upscale-casual quality: formal enough for paperwork, approachable enough for the dog park, with Sully as a built-in option for daily use.
Monsters, Inc. and the Sully Connection
James P. Sullivan (Sully) from Monsters, Inc. is one of Pixar's most beloved characters: a big, furry, blue-and-purple monster who turns out to be gentle, loyal, and fiercely protective of the child in his care. The Sullivan connection to Sully means that any large, fluffy dog named Sullivan is carrying Pixar DNA whether the owner intends it or not. Bernese Mountain Dogs, Newfoundlands, and large, shaggy mixed breeds are natural recipients.
The Irish Surname Line
Sullivan is one of Ireland's most common surnames, carried to America by emigrating Irish families in the 19th century. As a given name, it signals either Irish heritage, surname-name aesthetic, or both. It fits dogs that project a specific combination of warm personality and physical presence.
Formal-to-Daily Name Split
Sullivan registers formally, Sully operates daily. It's a common strategy for owners who want a distinguished full name on file without sacrificing practicality. Compare Lexington-to-Lex and Maximilian-to-Max for the same approach. The human name Sullivan follows an identical pattern in baby naming.