The Arabic word used here is "An'aam". Here is how Edward William Lane has defined it in his Lexicon:-
These types of livestocks are categorized by the Glorious Qur'an in the same way (i.e. four types of livestocks). The word can be used for specific types of livestocks (i.e. four types of livestocks mentioned by Lane's Lexicon and the Glorious Qur'an)
If you notice, you will see that Glorious Quran talks about these four animals only (and divided them into male and female) in Surah An'aam, verse 142-144:-
Surah An'aam, verse. 142:- And of the grazing livestock الْأَنْعَامِ are carriers [of burdens] and those [too] small. Eat of what Allah has provided for you and do not follow the footsteps of Satan.1 Indeed, he is to you a clear enemy.
Surah An'aam, verse. 143:- (Allah has created) eight pairs: two of sheep and two of goats.
Surah An'aam, verse. 144:- And (He has created) two of camels and two of oxen.
So, this makes it a total of eight livestock (Including the male and female of these livestocks).
Glorious Quran talks about these livestocks in the context of these four livestocks only, as lane's lexicon denotes that the word An'aam may specifically refer to these four livestocks. These four livestock can be further divided into two categories (i.e. male and female), thus making a total of eight livestocks.
Here is how Hans Wehr dictionary defines the word "An'aam". Notice how Hans Wehr also mentions four livestocks only and they can be further divided into two more categories (male and Female) making them a total of eight livestock:-