There are good reasons to want a specific initial. Breeders name a whole litter from one letter, some registries require it, and plenty of people simply want the name to match the one they already have at home.
Four letters carry more registered names than any others in this catalogue: S, M, B and C. Each list below is that letter's names in order of how widely they are used.
Every other letter
The catalogue filters on any initial, not just these four — the archive's Starts with control takes a letter and combines with everything else, so "female cat names starting with K" is one query rather than a search through a list.
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.