"Dyw hi ddim yn gyffrous."
Translation:She is not exciting.
A good opportunity to point out something about gender in Welsh grammar.
The mutation is caused by the word not the thing itself. If the word is masculine - see below - then the adjective will not mutate. An example of this would be
cefnder cyffrous an exciting cousin
regardless of whether the actual cefnder ('cousin') happens to be male or female. The usual arrangement, which applies to cefndryd and may well apply to gruffalos, is that the masculine word is used for males - and when the actual gender is unknown or unimportant. There may also be another word for the female
cefnither gyffrous an exciting (female cousin).
So what gender is gruffalo? It is English in origin, so it does not actually have a gender, as there is no such thing as grammatical gender in English. However it would probably be masculine in Welsh, for the same reason as cefnder. Its etymology is not known, but it is always possible that it was partly modelled on buffalo. This is of course ungendered in English, but it comes from the Spanish/Portuguese búfalo which is, again, masculine like cefnder with búfala being used for a specifically female animal. This is why you get, in Italian, Mozzarella di bufala - once you get milk from the animal you can assume it is female.
Another approach is to see how the book has been translated. From Wikipedia we find
|Language||Name||Soft mutation||Fem article||Gender|
The feminine singular article causes a soft mutation in all Celtic languages. I have given what the soft mutation of g would be so you can see it has not occurred and I have given the feminine singular article where it differs from the masculine singular. The Gaelic was not in Wikipedia so I got the gender from here. Esperanto La Krubalo may look feminine but it is not a gendered language - rather the feminine article was borrowed from French/Spanish/Italian as the ungendered article. Spanish has made the gruffalo rhyme with their word for buffalo, but Italian has chosen not to do this for some reason.
So there is total agreement amongst translators that the word is masculine. But the Welsh is actually Gryffalo not Gruffalo so it should be
Y Gryffalo Cyffrous
Anyone with any knowledge of Spanish, Portuguese or Italian automatically thinks of -o words as masculine and there are even more languages where -a is treated as feminine - all the above, + Latin, Greek, Russian (and other Slavic languages).
This means that when you hear a new word, perhaps borrowed from another language or made up, you just apply these gender rules. That's why pizza is feminine in all these languages. But in some other languages, new words often become masculine
Y pizza (m) Welsh
Am pizza (m) Gaelic
This really makes me (and I am sure a lot of other people) cringe when we hear it. I have also heard das Pizza (n) in German although it is officially feminine.
Almost exactly the same gender rules apply to bible (originally biblia in Greek) with the added irony that it wasn't even feminine in the first place, but neuter plural.
I can understand this happening in the distant past, but I would have thought that when pizza was borrowed in the 20th century enough people in Wales and Scotland would have realized that it was feminine for it to treated as feminine. But apparently not. It may yet come in. As the use of Spanish grows in the English-speaking world I suspect that it is only a matter of time before Welsh and Gaelic allow pizza to transition. D