How to write ‘filly’ in plural form of the word
1. The different versions of ‘Filly’ -
The german media suggests it’s ‘fillys’ (for the toy lines’ canon) and other advertisements of ‘Filly Funtasia’ (in spanish and english languages) call the creatures ‘fillys’ as well.
The german version is one that has the biggest number of kinds of merchandise so far, and when ‘Filly’ or ‘FF’ are mentioned in the articles with news, it’s usually that mention of how ‘Filly’ is a huge success in Germany. However, Dracco, the creator of the franchise, was never of Germany. So if you think ‘it’s a german toy line’, it may be an association with the former maker of its toys, ‘Simba’, who -didn’t- create the idea of ‘Filly’ in the first place.
The russian version often says the characters are ‘filly ponies’ or ‘horses’ and even the logotype of the magazine of rus. ver. is basically the words ‘Filly the little horses’ (”Филли лошадки”).
Merchandise made by ‘Multi-Pulti’, ‘Simba Russia’ and ‘Simbat’ (the plush toys) continues the tradition as well.
To add to that, ukrainian dub of ‘Filly Funtasia’ is mirroring this, calling the characters (who are the fillys) with the words that are like ‘ponies’ or ‘horses’ and the various synonyms of those.
I won’t be surprised if the russian dub of the show will have this detail for it as well, but again, it’s only possible -if- this dub will ever be created and reelased.
The russian fans already call the characters of the toy lines and the series as ‘horses’, with ‘Filly Funtasia’ being called “Филли Фантазия” which is ‘Fantasy’ - without the initial pun. It’s a coincidence that the ukrainian dub loses the wordplay as well, with its title being simply ‘Filly Fantasy’.
2. The origins of the words -
Why it’s ‘fillys’ in the german language? It may be based on how the plural forms are created in the language for other words, for example, ‘the ponies’ is written like ‘ponys’ in german (for ‘horse’, see ‘pferde’).
If it’s the case then it’s a very awesome thing since ‘Filly’ has an origin of its characters being based on the real horses in ‘Forest’ and ‘Beach Party’ canon. ^_^=
As for ‘toy line’s name plus Filly’ it may be ‘Filly Stars, Royales’ and etc. since german media calls the characters of ‘Filly Royale’ as such.
It can be a golden rule but I’m not sure it applies to each and every language because every one of them is different from others, but it can be true for at least english language. And it’s still possible to ask the creators on this matter.