Flater's answer is very good, and indeed many games have taken this path.
I would suggest a bit of variety though. Using suffixes and prefixes for different races can help you to emphasize the cultural and linguistic differences between your different races, partially explaining how they can have diametrically opposed viewpoints that bring them into conflict.
It can also tie in with interesting historical events within a single culture.
For example, there was a point in time where some of the well-to-do people from Liverpool began calling themselves Liverpolitans instead of the more prosaic Liverpudlians. From the Collins dictionary:
Liverpolitan (ˌlɪvəˈpɒlɪtən Pronunciation for Liverpolitan )
noun: a native or inhabitant of Liverpool
adjective: of or relating to Liverpool
Word Origin (C20 [20-th century?]): a supposedly status-enhancing adaptation of Liverpudlian from Liverpool + Greek politēs citizen
A very matter-of-fact scientific race may well make the noun, adjective and collective noun the same in the interest of uniformity. Alternatively, a totalitarian state may call individuals by the same name as the state as if emphasise that individuals can have no divergent interests.
However, on our own planet there are some cases where a prefix is used to distinguish between a country and its citizens. Take the country of Botswana, for example.
A individual from Botswana is referred to as a Motswana while the people from Botswana are known as the Batswana. Even though the official language of Botswana is English, around 90% of the population speak a language called Setswana.
Motswana in the Cambridge dictionary
noun [S] UK /mɒtˈswɑː.nə/ US /mɑːt-/
a person from Botswana
Batswana in the Cambridge dictionary
› people from Botswana
The wikipedia article on Setswana can be found here: Languages of Botswana.