My wife had horse ATG for aplastic anemia. That was during the time when horse ATG was routinely used in the U.S. and rabbit ATG was routinely used in Europe, without good data to show if one was better than the other. Her treatment may have kept her counts stable temporarily, but it didn't put her into remission, and she was later re-diagnosed with MDS.
In 2011 the National Institutes of Health (NIH)
published a study reporting that initial treatment with horse ATG was more successful than initial treatment with rabbit ATG in 120 patients with severe aplastic anemia. (They didn't study patients with
moderate aplastic anemia.)
For a patient already undergoing rabbit ATG treatment, they recommend waiting to see how well the patient has responded after six months. If they haven't had a good response by then, the NIH says that they consider the patient to be a candidate for horse ATG next, if it's available in their country.