YOU SAID:
Horses are being a kitten
INTO JAPANESE
馬は子猫になっています
BACK INTO ENGLISH
the horse has become a kitten
INTO JAPANESE
馬が子猫になりました
BACK INTO ENGLISH
the horse became a kitten
INTO JAPANESE
馬は子猫になりました
BACK INTO ENGLISH
the horse became a kitten
That didn't even make that much sense in English.