The primary era of known pre-historic alien habitation of the Alkonost Sector was that of the Precursors, a species of sentient beings who colonized several worlds in the sector in the second millennium BCE. Starting from approximately 1600 BCE, the Precursors built cities and, apparently, starports on Pazuzu, Zariya, Nadezhda and Pigulim; a few lesser structures and artifacts can be found in other star-systems as well.
Most of what is known about the Precursors come from their ruins on Pazuzu, which were studied extensively in the early 2500s. Another source of knowledge, which is somewhat less reliable, is from the Inheritors, who claim to have evolved on the Precursor home-world from local omnivores and thus claim ownership over all Precursor ruins.
From these sources of knowledge, it is known that the Precursors had, at their height, a technological level slightly more advanced than the pre-Scream Terran Mandate, and ruled, for the very least about nine sectors of space at their height. They had a radial symmetry and multiple tentacles which could be used for locomotion or manipulation, breathed an approximately standard atmospheric mix, and had a semi-rigid endoskeleton. Their cities and starships, like their bodies, had a radial symmetry. It is known that they practiced a form of psionics, though the details remain unclear as of yet.
The Precursors brought with them certain organisms of their original ecosystem, creatures which are now common throughout the sector in general and on Precursor worlds in particular.
Around 1200 BCE, the Precursors were wiped out within a single decade. While the evidence are inconclusive about the exact cause of their extinction, it is clear that their end was a horridly violent one, be that an invasion by another alien species or a particularly nasty civil war. Some of their structures are still radioactive to this day due to dirty weapons employed by one side in that conflict.
Once the Precursors were gone, there is little evidence for any sentient settlement of the Alkonost Sector... Until the Humans came, almost a millennium ago.