Sometimes the animal is decided and the species is not: a female pet is arriving and the name has to work either way. These are the names the records sex female regardless of what kind of animal carried them. For one species, see female dog names or female cat names.
How does this site know a name is female?
From how real animals carrying it were sexed in shelter records: female at 65% or more of a sample of at least five. Below that floor a name carries no gender rather than a guess.
Why do some very common names have no gender?
Because the registers that supply most of the counts record no sex at all. Gender comes from the shelter datasets, so a name common in licence data but rare in shelters can be well counted and still unsexed.
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 three 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. Nine of the eleven sources are North American, so read the ordering as naming in those places rather than worldwide. Browse the whole catalogue or read how the data is put together.