What does flowing hair mean?
That it has a sort of movement, it's a kind of wavy hair?
Esto es lo que dice el diccionario de la casa. Pero quizá sería más acertado "melena (que fluye) al viento".
Espera más sugerencias.
.. con su melena al viento...
I would go more for "melena suelta" I think.
"flowing" here implies "natural" so 'abundant and wavy' if your hair is wavy; 'long and straight(ish)' if your hair is straight(ish); and 'impossible to comb' if you wear long flowing dreadlocks.
... las crines al viento...
¿También se usa en inglés ' flowing' para las crines de un caballo?
Yes, flowing mane is a common phrase for a horse, but I don't think it has to be moving or blowing in the wind. From the oxford online dictionary:
(especially of long hair or clothing) hanging or draping loosely and gracefully:a long flowing gown of lavender silk
(of a line or contour) smoothly continuous:the flowing curves of the lawn
(of language, movement, or style) graceful and fluent:a flowing prose style
But most "flowing hair" does not involve wind or actual movement.
"Her flowing auburn hair framed her face",
"Her hair was long, flowing almost to her waist".
A horse with a flowing mane (or tail) could be standing perfectly still, in still air, or its long mane could be "flowing in the wind", if there was one.