Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
Filly is regular. It follows the -ies Conversion rule.
Use plural verbs: "The fillies are ready" (not "is ready").
Yes. The plural fillies is standard across all English variants.
The word filly follows the standard English pluralization rule. When forming the plural, filly becomes fillies. This follows the -ies Conversion pattern, which is one of the most common plural rules in English.
Understanding why filly becomes fillies helps with spelling and pronunciation. Nouns ending in a consonant followed by -y change the -y to -ies in the plural form.
When using fillies in writing, remember that it functions as a plural noun in sentences. It pairs with plural verbs and pronouns. For example: "The fillies are ready" (not "is ready"). This subject-verb agreement is essential for grammatical correctness.