I haven’t even checked plurals, I’m afraid, but I did have a nasty shock when I checked ‘cadno’ in the dictionary and found, as well as ‘llwynog’, that it can also be ‘mabyn’!!! I have forgotten all my Latin and have never learned old Norse, so I’m left wondering where all these words came from and whether every common animal has at least three names in Cymraeg!!! On top of that, if I had come across ‘mabyn’ in a sentence, I would have started from the idea that it has something to do with a son!!! Jackie
Yes. 
Llwynog in the north, Cadno in the South. (With the normal caveats on that sort of thing!)
I know “Cadno” is at least one form used round here, but as you say apparently “canddo” is a variant in the South for “Cadno”. I can’t help you there with how widely or where it is used, unfortunately - I haven’t talked about foxes enough to pick up on that! I did hold off on answering because I was going to ask in passing last night, but I forgot! 
The feminine of “Cadno” is “cadnawes” or “cadnöes”, apparently - but that’s dictionaries and not what I’ve heard.
When you say “given preference”, do you mean it appears first in a list? That doesn’t necessarily give it preference, I would say.
And you don’t necessarily need another commonly used word for the female of an animal. I need to make it clear that have no idea on the usage of this one, though!
The University of Wales dictionary also gives “madyn” (with a ‘d’) along with “madog” as possible words for fox, but says they could be taken from personal names, like “Reynard” in English (and other languages, presumably!)
I think that they might be somewhat less used, but that’s just a guess!
I think that was quite a common thing in all languages, before relatively recent standardisation, which has affected languages like English a lot more than Welsh!
And in answer to your query of where these words a from, here a a few possible derivations, from various reputable sources -
Llwynog - from “Llwyn”, bush, shrub, copse etc, + -og adjectival ending -
“Bushy”, referring to his tail.
Sounds a little -“eh? Bushy? Is that in Welsh as well?”
But “llwyn” as any bushy growth, eg a Woman’s hair in love poetry goes back to at least the 14th century in Welsh. Apparently.
Cadno - Oo, probably from a personal name! Like Reynard, Madog, Madyn. Well well.
Then “Canddo” became a variant. Because.
(There are other examples of this -dn- > -ndd- change in certain dialects, apparently. Including (so I read! " ‘soles’ = gwadnau> gwaddnau > gwanddau > gwandde".)
Madyn - There’s a Celtic word represented in Scottish Gaelic and Irish as “madaidh”, meaning wolf or possibly dog, with “madaidh ruaidh” [edit- ruaidh meaning red] occasionally (apparently!) meaning Fox, so it may be connected to that. Along with an apparently common tendency to give peoples names to a fox,perhaps meaning “mad-” tended towards the names Madyn, Madryn and Madog. Maybe.
So… third reply in a row, but bit more information!
A couple of seemingly reputable people describe “madyn” as an “obsolete” word for fox, so it’s probably just llwynog and cadno you have to contend with! (Canddo not really being that different).
[Don’t take what I say on this as certain, though!]
Not many when you consider how many different words there normally are/were for animals in different dialects in all languages!
Badger/brock,
Fox/Reynard/Tod/Tod Lowrie
etcetera, etcetera.
Personally I like the fact that dialects are alive and kicking in Welsh!
Ok, fourth reply in a row here, so maybe me just speaking to myself
But managed to bring this up in passing tonight.
The use of “canddo” for fox elicited basically blank looks from fluent Welsh speakers round here, although one did say something along the lines of it rang a bell so if you said it confidently enough he would have assumed it was dialect (which pretty much goes for a lot of stuff! ).
This doesn’t mean it isn’t used round here, and certainly doesn’t mean it isn’t used anywhere else, but I’ll be quite happy keeping to “cadno” and “llwynog”.
I tried to mention it in passing, but it turned into a bit of a conversation- and the North/south divide was (as is normal!) not quite as black and white as some sources (including me earlier ) will give.
Hope of some interest to someone!
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Diolch Owain bach! All that work for me, and me living in Yr Alban so very unlikely to be talking about foxes, or anything else, to any one!! I seem to have misread ‘madyn’ as ‘mabyn’ which shows the dangers of dictionaries!! I do watch a lot of S4C, so your efforts will help me when watching nature programmes. I bet Iolo Williams calls cadno llwynog!! from Jackie
p.s. I had failed to think of a single animal with different names in English, but I see what you mean now!!
Oh, you’re more than welcome - I enjoy this sort of thing!
So the work was at least as much for me!
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Many thanks for your inputs, it’s all very interesting.
Iolo williams book gives both names for fox,
and does the same for frog (Llyffant/Broga) .
But only one of the three names for a butterfly (not long before they start to reappear, hooray).
Cheers J.P.
Hwyl John, isn’t that pilipala? Or is that a children’s word like ‘flutterby’? Jackie.
Pili pala is one, my book has glöyn byw and i have heard iar - fach - yr - haf .
So we are kind of spoilt for choice.
Cheers J.P.
Wasn’t there once a long series of letters to the Guardian newspaper discussing the apparent fact that the name for butterfly is universally lovely in all known languages?
wel wel another mammal that has two names (well in my book).
Llygoden Ffyrnig / Llygoden Ffrengig - Brown rat.
Cheers J.P.
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Llygoden Fawr is also used. I use Llygoden Ffyrnig simply because it’s the most dramatic.
I remember it being a definite “Llygoden Ffyrnig” from my father, which is why I use it as well.
I like it the best, too- as you say, it’s the most dramatic one!
(Mind you, probably just a corruption of “Ffrengig”, according to stuff.)
Apparently (I claim no infallibility for the following statement)
“Another theory connects it [the etymology of the word “butterfly”] to the color of the insect’s excrement, based on Dutch cognate boterschijte.”
[Edit - Middle Dutch, apparently - but again, I know nothing on the matter!
@Louis ? ]
[And incidentally, I think that’s a rather marvellous word!]
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I heard that it was the other way 'round!
I’ve never heard of that word, but the Dutch for butterfly is “vlinder”, which I really love because it describes (to me, at least) the way butterflies dart about so beautifully. I’ll investigate that other word soonest, @owainlurch
"compare Middle Dutch boterschijte (“butterfly”, literally “butter-shitter”)). "
is in the article in that link!
Why not! Like I say, my father uses it so I’m quite happy to think of it as the original word!
Mind you, if the rat was introduced relatively recently from the continental mainland, the other form sort of makes sense.
I found it here: http://gtb.inl.nl/iWDB/search?actie=article&wdb=WNT&id=M010660 - it says it is Flemish. I only know “kapelleke” as a Flemish word for butterfly