"Cob" itself can refer to several things, and its gender doesn't typically change depending on the object's sex:
* Cob (bread): This is a type of bread and doesn't have a gender.
* Cob (horse): This refers to a type of male horse. There's no separate word for a female cob horse, you'd simply use "mare" or "filly" depending on its age.
* Cob (building material): This is a neutral term and doesn't have a gender.
If you are looking for a word for a female horse of the cob type, the correct terms are "mare" or "filly".