The Red Guard is the variant name used by the "Persean Democratic People's Revolutionary Council" (i.e. the "communist_clouds") faction's ships. Their fleets frequently consist of ship variants like the Red Guard Hammerhead - which is basically a normal Hammerhead with a new red paint job. The faction itself is one of the colonial competitors that occasionally sends out fleets to colonize other planets. The colonial competitors feature is optional though, so you have to enable it for a chance to see them and their Red Guard ships. (And that feature was disabled by default in older builds of this mod, so you might have never seen them before...)
With that being said, I'm not sure the "Red Guard" name is actually used anywhere other than in the ship variant's name, so I probably should have called them "Revolutionary Council ships" for clarity. Sorry for any confusion...