While it can technically be used (very, very carefully) as a herb, the arnica for pets were talking about is a homeopathic remedy. In fact, arnica either in whole plant form or large amounts of undiluted arnica (for example, plant extracts or oils) is poisonous and you definitely dont want to give it to your pet. Homeopathic arnica, however, doesnt have that same issue. Why is that? Well, with homeopathy, remedies are diluted to make them more effective.