It actually is prehistoric. One of the oldest plants around. Equisetum arvense. As to how to get rid of it, if I knew that I would make a fortune.

Here is a link that might be useful: Equisetum arvense

You can get rid of it but *only* if you spend several years removing every bit every time you see it. And it won't be fun dealing with it in a rock garden.

By constantly removing it, you will eventually starve out the root system.
If you relax for just one season, the horsetail will set you back to square one.

Resources for you:

1. âÂÂField Horsetail and Related Speciesâ (Oregon State University; publication # PNW 105) describes the various kinds of horsetail, and the preferred management. http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/20760/pnw105.pdf

2. âÂÂHorsetail Scouring Rushâ http://www.co.thurston.wa.us/health/ehipm/pdf/horsetail.pdf The weed killers mentioned in this publication are for use by professional pest control companies.

Here is a link that might be useful: 1st reference, above