Ythrians

Of the twelve works collected in Poul Anderson's The Earth Book Of Stormgate:

the first is set on Ythri, in fact is about First Contact with Ythrians;

the second, also about human-Ythrian interaction, is set on the planet Gray, later renamed Avalon;

the tenth involves Nicholas van Rijn traveling in an Ythrian-crewed spaceship;

the eleventh and twelfth are set on Avalon after it has been jointly colonized by human beings and Ythrians.

Further:

the new introductions and conclusion in the Earth Book are written by an Avalonian Ythrian;

three installments later in the History of Technic Civilization, the novel, The People Of The Wind, is set mostly on Avalon, which, despite its human majority, has become part of the Domain of Ythri and resists annexation by the Terran Empire;

five installments after that, The Day Of Their Return involves an Avalonian Ythrian spying for both Domain and Empire.

Thus, although a linear summary of the History presents a chronological sequence of human institutions, League, Empire etc, Ythrian interaction with humanity is also a major feature of the History, comparable to the role of the Merseians, who also have human beings living within their realm.