Space Pirates-A threat ranging from mostly trivial to serious.  On the “mostly trivial” side, we have cargo thieves on stations, whom hack the cargo handing systems to reroute valuable cargoes to their ships.  Going from there, there is some idiot that gets ahold of enough weapons to arm a freighter and start “living the dream”-until a naval warship blows them up.  Smart idiots do this far enough away from the major powers that they can get away with this for a long time.  Usually until they rob the wrong ship and somebody hires a PMC to find them and blow them out of the sky.

Continuing the escalation is actual “privateers” in the form of military ships (usually griffin or zebra) that found themselves on the wrong side of politics.  This especially includes griffin ships, with nobles that have been convicted of treason by the Moot using their resources to get out of the Empire as fast as possible.  Some actual pirate “kingdoms” have resulted from this, with the Razorbeak Commonwealth being the largest.  And, of course, Changeling Queens raiding worlds for emotions and memories to devour for their Empresses.

Prior to the Horizon War, most naval actions were based around piracy/privateer suppression.  This was one of the major reasons for the existence of the Princess Luna-class fast battleship and the Kestrel-class fast battleship (having the acceleration profile of a battle cruiser with the weapons of a battleship was vital for bringing pirates to heel).  After the Horizon War, the vacuum created by the massing of battle fleets was filled by…entrepreneurs of all types and sorts.  Which explains why the Royal Equestrian Space Force is building the Princess Sunset Shimmer-class fast battleships.