Space Marine-Oddly averted by the Royal Equestrian Space Forces, mostly because they never really saw the need for such a force. If they need ground troops, they get them from the Royal Equestrian Ground Forces, usually in the form of a Reaction Battalion on a Bright Bulb-class GFSS (Ground Forces Support Ship). After the Horizon War, the RESF and the REGF began to work out force sizes for ships, with a proposal for company-sized units aboard cruisers, reinforced-company sized units on battlecruisers, and battalion-sized units of infantry on capital ships.
Embraced with glee by the Imperial Griffin Marine Corps, whom will fight with anyone over the title of “The Emperor’s Finest“. The IGMC can get away with having their crews on Imperial Griffin Space Navy ships by the simple fact that a Marine is replacing an equivalent naval rating, as well as providing ground force support. Since every single Marine wears a suit of powered battle armor and is backed up by a veritable swarm of drones, orbital fire support, and the other Marines to their flanks, this makes an IGMC Marine (NEVER call an IGMC Marine a “soldier” unless you want to get punched) a dangerous foe indeed.
The Senate and Minotaur Republic Forces doesn’t recognize any difference between a spacer or a soldier. Basic training is infantry training, every soldier in the SMRF from new recruit to Dux is required to demonstrate basic infantry proficiency at least yearly, and every single soldier in the SMRF legions has an infantry rifle somewhere handy. While they do have specialist ground and boarding soldiers in powered armor for those missions that require it, the SMRF doctrine of “every soldier is an infantry soldier” has served it well on a number of occasions.
The Zebra Kingdoms espatiers are in the mold of the IGMC, but like a lot of things in the Kingdoms they have to contend with issues of too few bodies for too many missions. Especially if Capital has to act against one of the larger noble houses, careful alliances are needed to generate the number of troops needed to bring down a noble house.