In the 41st Millenium, Equestrians have explored the entire Galaxy, and colonized thousands of planets.
Surrounded on all sides by enemies, they were forced to give up the ways of peace, and to fight for their continued survival.

Please note that "MAREINES" is not a spelling mistake, it is a combination of the words "Mare" (Female Pony) and "Marines"

Mare + Space Marines = Space Mareines