To date, humanity has only encountered species vastly inferior to itself in technology, vastly superior, or extinct.

In addition, the majority of alien sentiences encountered have proven so alien as to make communication difficult, and in many cases pointless.  Many aliens inhabit ecosystems which humanity has difficulty even accessing, with gas giant civilisations being common, self-sustaining electromagnetic pattern based life inhabiting solar transition regions, ultra-slow intelligences evolved to low energy environments and entities adapted to existence in a vaccuum.  As a rule, the further the habitat from one humanity could exist in, the less likely meaningful communication.

That said, from the perspective of the average human citizen the average alien is not that different to that average human.  This is because humanity tends only to interact significantly with those races most similar to it.

Notable races, which include races engineered by humanity itself, include:

Aslan, a chimeric species created by blending human and animal dna

Hivers, ambassadors from a post-singularity culture created in order to interact with humanity

Newts, the most human-like species yet encountered

Pacifiers, ancient automated galactic defence vessels

Vargr, uplifted Terran dogs lacking natural manipulatory organs

Vegans, primitive hunter-gatherers given technology by the Imperium