While playing Mount and Blade Warband First Manassas mod, I joined up with the 14th NYSM, also known as the Red Legged Devils. (Thank you Jackson for that cool nickname)

I learned a lot about the regiment and how they even had a rival in the Southern Army. The Wheat Tigers.

Ever since Bull Run where the 14th fought the Tigers in a brutal melee and forced the Tigers back, they were rivals throughout the course of the war.

My question is this. Were there other regiments that had rivals on the opposite side?

4678665
"Red Legged Devils"? Zuaves?

Honestly I'm not sure, I've heard nothing like that for the 173rd NYSVI or the 165th NY.

4678682 14th

4678731
Not Zouaves, but still had the red pants... and drawn from the same area as the 173rd

4678752 yeah. It's confusing at times. Not all Zouaves had red pants

In the fog and confusion of war, it was often impossible to clearly see who you were fighting, let alone make out the flags of your opponents and "know" that you had faced them before. Certainly, distinguishing characteristics helped: More than one Confederate remarked in their journals that they knew that they were up against "the Black Hats," the name they gave the Iron Brigade, and more than a few cursed under their breath when "those damn green-coated tree frogs" (my United States Sharpshooters) appeared in their front and began sniping at them. In a few records I read soldiers who were talking with prisoners from either side would be happy to encounter soldiers that they had fought before—both sides shared a morose sort of compatriotism. They called it The High and Honorable Society of Them Who Have What Been Shot At.

The vagaries of war being what they are, it is hard to find pure examples of one side singling out individual units on the opposite side and knowing them as "a rival." When individual units could be swept away in a single battle, and how difficult it was to command specific units into line against one another, the enemy was whoever you faced that day across the smoke-strewn fields.