Counterpart races are 2 races, on both factions, that, pretty much, reflect each other - whether it is culturally or conflictual. I'll give my list as to who i think those races are:
Orcs vs Humans
Probably the most iconic counterpart races.
Their relationship extends back to Warcraft 1: Orcs and Humans, when they first clashed. Ever since, their hatred towards each other remained the quintessential foundation for the factions' rivalry. Not to mention that they are both the leader personalities, and founders, of their respective factions. Orcs founded the Horde, and the Warchief position was always the highest authority within that faction. while Humans founded the Alliance, and their position as High King was always the utmost authority within it.
Gnomes vs Goblins
Both being diminutive, technology-obssessed creatures, their rivalry in that department has become famous, to the point of Hearthstone having an expansion about it. Even the Engineering profession has either Gnomish engineering or Goblin engineering. while the Gnomish approach to engineering is that of structure and calculation, the Goblin approach is all about amateurish, malfunctioning design.
Worgen vs Undead
Ever since the Cataclysm, the Worgen of Gilneas and the Forsaken Undead have been in a constant war between each other, over territory. But, it's not just the conflict between them, but also their similar theme. While Forsaken Undead have been afflicted by the curse of undeath, Gilneans have been afflicted by the curse of the Worgen. They're both dark races, as can be attested by their racial abilities and cities.
Huojin Pandaren vs Tushui Pandaren
Same race, different philosophy. While Tushui Pandaren are passive and contemplative, Huojin Pandaren are active and impulsive.
Kul Tiran Humans vs Zandalari Trolls
As of Battle for Azeroth, they are in a conflict with each other. The Siege of Dazar'alor is a testimony to that. So deep is their hatred, that Talanji could not stand the presence of Derek Proudmoore among the Horde. But, it is not just the conflict between each other. They also share a common denominator. Both of these nations have an impressive fleet on their side, and they were recruited for it to their respective factions.
Mag'har Orcs vs Lightforged Draenei
Although not directly in conflict with each other, the Lightbound Draenei declared war on the Mag'har, in their recruitment scenario. Ever since the Legion left their world, and without a common enemy to fight against, the Alternate Draenor Draenei became so obsessed with the light, to the point that they want to convert everybody else to their precious light. On the other hand, the Mag'har refuse to be converted. they believe the light will doom them all, while the Lightbound Draenei believe that the Mag'har will transform the land into a desolate landscape. Both of them being very army-oriented. While the Lightbound Draenei are more crusader type of soldiers, the Mag'har Orcs are more barbaric kind of soldiers.
Void elves vs Blood elves
High elves and Blood elves have bad blood between them. While High elves are part of the Alliance, the Blood elves turned to the Horde. Their animosity towards each other was amplified by the Purge of Dalaran and it can be seen on the Isle of Thunder. Although, Void elves are not named High elves, their population consists of former High elves and Blood elves. They are, basically, the High elf race on the Alliance. They just couldn't give us them, because of their, almost, identical identity to the Blood elves. plus, Void elves are not welcome in Silvermoon, as the power of the Void could corrupt the Sunwell. They had been exiled for delving in the Void. So, there is bad blood between them as well. Aesthetically, they contradict one another, as the Void elves are very dark-looking, while the Blood elves are very bright-looking.
Night elves vs Nightborne
After listing Blood elves vs Void elves and Huojin Pandaren vs Tushui Pandaren, i can't dismiss Night elves and Nightborne. They both have 'Night' in their names. Their relationship is similar to that of the Highborne and Night elves, as the Night elves prohibited Arcane magic for a long time (until Cataclysm) and absolutely despised the Highborne for their actions, during War of the Ancients. Although, there aren't any animosities between the two, they still hold grudge against each other. Magistrix Elisande called Night elves "Trolls, skulking about in trees" (given, she is not part of the playable faction), and Tyrande told Thalyssra how she didn't trust her (given, they did accept Highborne back into their society during Cataclysm).
I was thinking about mentioning Night elves vs Tauren, as they are both nature-oriented; or Night elves vs Blood elves, as they are opposites (Moon vs Sun); or Draenei vs Blood elves, as they had a conflict during early stages of the Burning Crusade; or Draenei vs Orcs, as they were in a conflict on Draenor; or Night elves vs Orcs, as in the case of Ashenvale; or Night elves vs Undead, as in the case of War of Thorns; or Dwarves vs Tauren, as in the case of Camp Tourajo and their, respective, connections to the earth; or Undead vs Humans, as the Scourge was a major threat to the Human Kingdom; But, i wasn't sure about these.
What do you, guys, think about it? am i right? am i wrong? post your counterpart races and explain why you think so.