I could not find any grass or plant called equestrian grass. There is one called horsetail and its genus is Equisetum. That is probably your mystery plant. It is invasive in wet areas. If kept a bit drier it is not so bad. You can also install underground barrier walls with a thick plastic or metal flashing material. For more info go to the following web site and type Equisetum in the search box.
Q&A Library Searching Tips