* "Baby" is a human term: Horses have foals, not babies.
* No special names for mothers: There isn't a specific term for a horse that has had two foals, like there is for a human mother with multiple children (e.g., "mother of two").
However, you could use terms like:
* Mare: A female horse, regardless of how many foals she has had.
* Dam: A female horse that has given birth to a foal.
So, while you might say "that mare has had two foals," you wouldn't use a specific term for a horse based on the number of foals it has had.