Horses may reject their babies, called foals, primarily due to a failure in the mare-foal bonding process that normally occurs immediately after birth. This bonding is crucial and is initiated by the mare smelling and licking the foal. There are several reasons why a mare might reject her foal:

• Disrupted maternal bonding : Interferences such as excessive human interaction, abnormal or difficult deliveries, or removal of the placenta too soon can hinder the mare's natural bonding process with her foal right after birth, especially in the first six hours, which is a critical period.
• Mare-related factors : Inexperience (particularly in maiden mares), stress, pain (such as mastitis), or hormonal imbalances involving progesterone, prolactin, and estrogen can cause maternal rejection. Some breeds, like Arabian mares, may have a higher genetic predisposition to rejecting foals.
• Foal or environmental issues : An abnormal or sick foal that does not naturally stimulate the mare's maternal instincts, changes in the mare’s environment, or rough handling may also cause rejection.

Mares that reject their foals may show disinterest, fear, or aggression such as kicking or biting the newborn. When aggressive rejection occurs, intervention may be necessary to ensure the foal's survival, including raising the foal as an orphan or using a nurse mare.

In summary, foal rejection is generally linked to a failure in early bonding due to hormonal, genetic, environmental, or experiential factors affecting the mare’s maternal behavior.