here) but also made anticipatory references to two alien races that had already appeared but in installments set later in the fictitious history.
"...oxygen-breathing mercenaries, human, Merseian, Gorzuni, Donarrian - adventurers, the scourings of space..."
-Poul Anderson, Rise Of The Terran Empire (New York, 2011), p. 181.
(I am reminded of a Doctor Who episode that contained the memorable dialogue: "You should have seen them, the scum of the galaxy!")
As already noted, Merseians and Gorzuni had been introduced in earlier installments. A Donarrian had appeared in "Sargasso of Lost Starships," published in 1952 but set centuries later. This post about intelligent quadrupeds lists three Donarrians and also three Wodenites although it misses the Wodenite Nadi who appears in "Lodestar" and Mirkheim. The travelers in A Game Of Empire would have visited a Donarrian settlement on Daedalus but, unfortunately, their itinerary changed.
The other race anticipated in Mirkeim is the Ymirites. I think that otherwise they are mentioned only in "Hunters of the Sky Cave"/We Claim These Stars!, which is where they appear. Since the Baburites are hydrogen-breathers, Anderson's characters naturally wonder whether the hydrogen-breathing Ymirites are also involved if only in conflict with Babur. However, the Wodenite Adzel points out that Ymirites, inhabiting Jovian planets, are as unlike Baburites, inhabiting a subjovian planet, as they are from oxygen-breathers inhabiting terrestroid planets.
This does not prevent the organizers of a secret army from hinting to their mercs that the Ymirites have an interest... Disinformation: the Ymirites, also suspected in "Hunters of the Sky Cave," genuinely have no interest in the activities of inhabitants of small planets.