Kingsley has a regal quality that stops just short of the obvious King — it's the same authority register but with three syllables and a surname structure that feels more considered. At rank 1306, it's a choice that appears on dogs whose owners wanted something that implies status without resorting to the blunter vocabulary of Rex, King, or Prince.
The Surname-Aristocracy Register
Kingsley is an Old English surname from the place name "king's meadow," and it carries that pastoral-aristocratic quality that certain British-inflected names do well. English Setters, Cavalier King Charles spaniels, and any breed with a certain imperial bearing are natural Kingsleys. The name implies a dog who accepts your authority while somehow communicating that it finds your authority charming but not entirely binding.
Sir Ben Kingsley and the Film Connection
Sir Ben Kingsley — born Krishna Bhanji, the name Kingsley comes from his father's middle name — is one of the most decorated actors of the last 50 years, with an Oscar for Gandhi (1982) and a career of remarkable range. Owners who arrive at the name through that association are giving their dog a quietly heavyweight cultural reference. The human name is at /names/kingsley.
The Counter-Reading
Kingsley is three syllables with a grand suffix, which means the full form sounds better in introductions than in daily commands. Kingly would be an odd shortform; King is available but flattens the name considerably. Most owners who use Kingsley just use the full three syllables and find the commitment doesn't feel burdensome. Compare Winston or Pemberton for similar register.