What is a "filly" horse?

Question

I read eight belles was a "filly" what does that mean?

And a filly does not usually run in the Derby is that true??

Answer 1

Filly - A female horse or pony not yet full grown.

Colt - A male horse or pony not yet full grown or gelded (castrated).

Mare - A fully grown female horse or pony.

Answer 2

It's a young female horse under 4 years old

yeah, they usually run in the kentucky oaks with the other girls.... but fillies have won the kentucky derby before