During its history Cybertron has encountered vast numbers of alien species. For the sake of distinction, sapient aliens are here referred to as xenos. Cybertronians generally see xenos as being divisible into two sorts: xenorganic and xenomechanic. 

Xenomachines are sentient machines not built on Cybertron or its colonies. During the Golden Age, Cybertron had excellent relations with several xenomechanical races, and fought for and alongside several xenomecha races in various military actions. Some xenomechanisms fought alongside and for Autobots or Decepticons during the Third War. They are variously called xenomech, xenomecha, xenomachines, xenomechs or xenachines.  

Xenorganics are sentient non-mechanical or partially-mechanical beings by definition not native to Cybertron. Cybertron's relations with xenorganic races were rather rocky in the early and middle Golden Age and deteriorated rapidly after the Great War started. A large percentage of the Cybertronian population felt threatened by xenorganics. Most states restricted the areas xenorganics could enter, with the Legascion states adopting the most extreme position and outright banning any xenorganic entry into any of their territory.

The spectre of Cybertronian slavery under xenorganic rule hovered over the cultures of both Autobots and Murions. The constant nudging reminders in every cultural medium from horror cinematics, future distopia novels and high-emotion art to everyday serials and news reporting maintained a sense of "us against them" that made Cybertron a suspicious and unwelcoming place to most xenorganics. Violent crime against xenorganics was common in several states, to the point that Praxus in particular simply refused to police crime against xenorganic visitors because they were so universally hated.

Hostility towards xenorganics did nothing to relieve the religious tensions between the Primordialist north and Sigmite south, with "organic worshipper" being a common and vicious insult from members of the former to the latter. The attempt by a group of xenorganic tourists to enter the Prima Palatia, the frenzied bludgeoning and riot which followed [leaving the tourists in such poor shape that the TEC were only able to identify two of the seventeen], emphasized just how threatening and frightening the average Cybertronian found xenorganics. It was observed in more than one Iaconic newsweb that only the spectacle of "foul-slimed, tentacle-waving bags of puss within the hallowed walls of our beloved Palatia" could provoke such bloodlust from Iaconics. That this language was not considered outre shows just how revolted most Cybertronians were by xenorganics.

A series of detailed studies by social psychologists in Nova Cronum and the Neutral Territories proved that all Cybertronians have a loathing of certain organic attributes that is so fundamental and so widespread that it could only have come from either Vector Sigma or from Primus, depending on one's views. The studies proved that the reactions were so specific that they were able to zero in on a particular shape and even colouration that triggered the hostility. The study showed that 93% of test subjects had a vehemently hostile reaction to a simple depiction of a bulbous, tentacled alien, rising to 98% when the depiction had green spots or similar features such as eyes, whereas only 18% of the same subjects had a hostile reaction of a simple depiction of a slender alien with rigid, jointed limbs. The study went on to posit that this green-eyed, tentacled bulb could in fact represent the true form of the Builders, spawning debate that it would be charitable to call heated. 

Xenos often appeared in Cybertronian culture.  

Xenos species and races: 

Magneticks

Tankworms

On interactions with xenos:

Xenophilia

Xenopublic