The Thuja Green Giant is known to be toxic to animals like dogs, cats, and horses. Due to the neurotoxin, Thujone, consumption can cause mild symptoms like irritation and itching if it comes in contact with skin. Symptoms can become severe if ingested and range from diarrhea, muscle spasms, or organ failure in extreme cases. This type of tree is especially toxic to horses, so we advise that you avoid planting it around stables and paths that you take the animal on.