Today while reading news I came across this line :
India defeated Bangladesh to reach the champions trophy 2017 final where it will face arch-rivals Pakistan.
I doubted whether it will be arch-rival or arch-rivals so I googled for the word and found another sentence on oxford learners dictionaries website :
a game between Rangers and their arch-rivals Celtic.
Merriam-Webster quoted this example
In baseball, the Boston Red Sox and the New York Yankees are archrivals. ( Yes it was without hyphen)
For me the arch-rivals in third example seems fine as we are taking about two teams( two subjects are referred , indicated by are too that we are referring two subjects) which are arch rivals for each other. But the arch-rivals in first two example doesn't sounds all right, in first and second example we are talking about India's and Ranger's opponent (single subject) so shouldn't be arch-rival used there in place of arch-rivals ? Or it is because we are referring to collective noun( team) we have to use arch-rivals ? Or is it we always use arch-rivals everywhere ?
Another question, is it okay to use it without hyphen or its an error in Merriam-Webster ?