Thanks for your question to Ask Extension! Horsetail Reed is a member of the horsetail family of Equisetaceae. Equisetum arvense and Equisetum hyemale (scouring rush) are common forms. It is an aggressive plant and once established, is very difficult to get rid of. Not only is the root system deep, it also propagates by underground runners and when these are broken each part produces a new plant; this makes digging them out a tedious effort which must be continued each year; tilling is insufficient.
See page 58 at this link: http://www.louisvillewater.com/sites/louisvillewater.com/files/user_uploads/Water%20Quality/Wellhead... for a photo and short paragraph on toxicity to horses; note that this is a list of 'unwanted' plants. For a good summary about planting/care/toxicity see: http://homeguides.sfgate.com/horsetail-plant-care-41513.html . Due to its aggressive growth and ability to 'take over' a flower bed, it is best to contain Horsetail in a pot or similar barrier...such as planting the 'pot' in the soil with its lip just about ground level. Several references to Horsetail concerned questions about how to remove/destroy it, so be careful! It can even invade a neighboring yard at the fence! With this thought in mind, you may wish to check your neighborhood's association for any guidelines. I hope this helps and if you have follow-up questions, please let us know!