179 of the 395 names beginning with J are ones English Wiktionary also lists as a given name for a person. That is comfortably the highest proportion of any letter, and against a catalogue where the overall share is about a quarter it makes J the letter where people most often gave their animal a person's name.
Jake, Jasper, Jackson, Joey, Jack
The top of the list reads like a school register, and the letter is used accordingly: 82 registrations per name, above the catalogue figure. Human names are the most heavily used class in this data generally, and J is where that class is most concentrated.
Almost no thing-nouns at all
Compare it with B, which is full of colours and objects. J has essentially none. There are very few English nouns starting with J that anybody would hang on an animal, so the letter had nowhere else to go — the vocabulary decided this, not the owners.
Short and level
90 of the letter is a single syllable — Jake, Jack, Jax, Joy — and 153 measure male against 125 female, close enough to even that the letter carries no reading of its own.
What are the most popular pet names starting with J?
Jake, Jasper, Jackson, Joey and Jack lead, ordered by registrations across eleven public registers.
Why do J pet names sound like people's names?
Because they mostly are: 179 of the 395 names starting with J are dictionary-listed given names, the highest share of any letter. English has almost no J nouns to use instead.
Are J names good for a dog?
They are used more than the catalogue average, at 82 registrations per name, and about half of them are one or two syllables — which is the shape that works when a name has to be called.
Every count on this site is a tally of real registrations: pet licences and shelter intakes published as open data by New York, Seattle, Austin, Montgomery County, Cambridge, Zurich, Basel and four Australian councils. Popularity here is the average share a name holds across those registers, so a name common in a small city is not buried by one common in a large one. Five of the eleven registers are North American and they supply 86% of the records, so read the ordering as naming in those places rather than worldwide. Browse the whole catalogue or read how the data is put together.