* Gender: If you're talking about the gender of a horse, the opposite of "filly" is colt.
* Action: If you're talking about the act of filling something, the opposite of "fill" could be empty, drain, or evacuate. However, "filly" doesn't directly refer to an action, so this isn't a perfect fit.
* Age: While not directly opposite, "filly" refers to a young female horse. The opposite in terms of age would be a mare (adult female horse).
To give you a more precise answer, please provide some context about how you're using the word "filly."