When we left our part of the Beta quadrant to escape the carnage and go further in with the help of the Gateway, we knew that we would meet other civilizations, other aliens. No Federation ships, or probes, as far as we know went this far. We compare our situation as the same then in the time of Captain James T. Kirk exploration into deep space, something the like "new neighbors" always were the possibility to meet, bad or good. Of course, when we evacuated our old sector of space, we had on board other aliens. Some are part of the crew right now, and of course, after 7 years of exploration, we did meet others and many are still a mystery to us.
Like you, I'm sure that this corner of the quadrant as a lot more surprises in store, but here is who we have so far. You can browse the Alien database below or the AVL, Aliens Visual List.