Mares can become aggressive during pregnancy because they are uncomfortable or because of the hormonal changes that are taking place. If your mare’s aggression mimics that of a stallion, then it is possible that your mare is suffering from granulosa theca cell tumors (GTC). These tumors produces steroids and in most cases male testosterone. Testosterone is the male sex hormone and will cause your mare to act like a stallion.
These tumors are located by performing an ultrasound scan and then blood work will be done to confirm the presence of testosterone.