What is the official name for a horse that young female?

There isn't an official name for a young female horse, but filly is the most common term used.

Here's a breakdown of horse terminology:

* Filly: A young female horse, typically under four years old.

* Mare: A female horse four years old or older.

* Colt: A young male horse, typically under four years old.

* Stallion: A male horse four years old or older.

* Gelding: A male horse that has been castrated.

So, while there's no official "name" for a young female horse, "filly" is the standard term used to describe one.