Here's why:
* Foals are typically too young and inexperienced: At that age, they are primarily focused on nursing and learning about their surroundings. They haven't yet developed the strong sexual urges that would lead to mounting behavior.
* A pregnant mare is physically uncomfortable: The mare's belly is swollen and her movements are limited, making it difficult and even painful for a foal to mount her.
* The mare's hormones: A pregnant mare's hormones are changing, and she may not be receptive to sexual advances.
While it's possible for a colt foal to show some exploratory behaviors related to mounting, it's highly unusual for them to attempt it with their mother, especially when she is pregnant.
Important Note: If you observe any unusual or aggressive behavior in a foal or mare, it's crucial to consult a veterinarian or an experienced horse professional. They can help determine the cause and advise on appropriate management strategies.