I remember seeing outside a veterinary practice a sign including the words ‘Catz and Dogz’ (or vice versa). I didn’t remember the name of the practice, but I knew exactly where it was/is, so was able to search by location online. The practice’s website contains no mention of ‘Catz and Dogz’ and indeed mentions ‘Boarding Dogs and Cats’, but mentions ‘groomz Pet Salon’. Even if my memory of ‘Catz and Dogz’ is faulty, the linguistic issues it raises – pronouncing and spelling final ‘s’ as ‘z‘ – are still valid.
I suspect that many native speakerz of English do not realize that the pronunciation of the ‘s’z on the endz of most plural nounz is actually ‘z’. I didn’t until I starting reading semi-seriously about linguistics. (Or maybe not even then. I didn’t consciously know about the similar alternation of ‘d’ and ‘t’ in past simple verbs until the class component of my first TESOL qualification.) But some wordz stay with the ‘s’ pronunciation. If you say ‘Cats and Dogs’ very slowly and carefully, you will hear that ‘Dogs’ is actually pronounced ‘Dogz’ and ‘grooms’ as ‘groomz’, while ‘Cats’ stays as ‘Cats’.
The key is the last letter of the original word (or more accurately the last phoneme). All English phonemes are either voiced or unvoiced. ‘Voicing’ is a buzz of the vocal chords as the sound is produced. You can feel this if you hold your fingers lightly against your throat as you say ‘zzzz’ (voiced) compared to ‘ssss’ (unvoiced), or hear it if you place a finger lightly in each ear. (Possibly more so for men, with our longer vocal chords.) In English, the voiced phonemes are all the vowels (speakerz), /m/, /n/ (nounz) and /ŋ/ (as in ‘king‘, /b/, /d/ (endz) and /g/ (dogz), /v/, /ð/ (as in ‘this and that’), /z/, /ʒ/ (zh) and /dʒ/ (j), and /w/, /l/, /r/ and /j/ (as in ‘you’), and the unvoiced ones are /p/, /t/ and /g/, /f/, /θ/ (as in ‘thick and thin’), /s/, /ʃ/ (sh) and /tʃ/ (ch), and /h/.
It is relatively difficult for the vocal chords to switch from a voiced phoneme to an unvoiced one, and vice versa. Try ‘sssszzzzsssszzzz’. It is quite unnatural to say ‘Dogs’ (that is, with a ‘s’ at the end). It is much easier to keep the voicing ‘turned on’ and make the ‘s’ into a ‘z’. This process is called assimilation, and occurs in a number of contexts in a number of languages. It is probably even more unnatural to say ‘Catz’, as ‘t’ and ‘s’ are both unvoiced, so there’s no reason to turn the voicing on. (‘Dogs’ (with a ‘s’) is likely to turn into ‘Docs’, and ‘Catz’ into ‘Cadz’.) A similar process happens with the ‘apostrophe s’s of possessive nouns and ‘s’s of third person singular present simple verbs. Those, and various other forms of assimilation, are more complex than that, but for present purposes, I’ll leave it there.
A search online for ‘Catz and Dogz’ shows various usages, including a pet boarding establishment near Perth, Western Australia and a pair of nightclub DJs in Berlin, Germany and Szczecin, Poland. It also includes a reference to ‘Petz (Dogz and Catz)’, which is ‘a series of games dating back to 1995, in which the player can adopt, raise, care for and breed their own virtual pets’. The developers seem to have gone overboard with the ‘z’ spelling: ‘The player starts at the Adoption Center, where he/she may choose a Dogz or Catz to adopt of a Breedz and gender of their choice. Once the player has found a Petz, the user can adopt and name the new Puppyz or Kittenz … There are a number of Toyz … and Treatz available … Other animals include Pigz and Bunnyz’. Their own website also refers to ‘Dolphinz’.
‘breed’, ‘puppy’, ‘kitten’, ‘toy’, ‘pig’, ‘bunny’ and ‘dolphin’ all finish with a voiced phoneme, so the standard plural ‘s’ *is* pronounced ‘z’, but ‘pet’ and ‘treat’ finish with an unvoiced phoneme, so the standard plural ‘s’ remains pronounced ‘s’.
Other ‘z’ spellings which spring to mind are: Wrapz (unvoiced, therefore unnatural; a snack bar in Castlereagh St, Sydney, apparently part of a global chain, also a line of eyewear); the 1970s-80s New Zealand pop group Split Enz (voiced, with and without the ‘d’, therefore natural); Bratz (unvoiced, therefore unnatural; their website (see what I do in the interests of research!) includes the words ‘Action Heroez’) and Dollz (voiced, therefore natural). I’m sure there are many more. Allianz Insurance doesn’t count: it isn’t an English plural noun, and apparently is it pronounced with a ‘s’ at the end, as per the original German pronunciation.