Algae, especially blue-green algae, may be potentially toxic to horses. Certain conditions increase the release of toxins from algae. If there are fish in the pond you will see fish be affected first as they will start dying. Other small animals or birds that obtain water from the pond would be the next to be affected. The key word is potentially. There have been very few reported cases of horses being affected by algae toxicity. Effective and safe algaecides include PAK-27 manufactured by UAP Timberland and "Green Clean" manufactured by Bio Safe.