Horsetail is a one-ingredient dietary supplement in the form of a dry infusion tea. Ponytail - learn more The horsetail plant ( Equisetum arvense ) is also known as fir or fir. This is due to its appearance, which resembles a miniature fir tree. Horsetail can support remineralization of the body. The silicon it contains helps keep your hair, nails and skin in optimal condition.