When it first came out, there was a small debate on the real name of agar.io. The URL for the site clearly listed it as "agar.io", however, the ".io" was also a visible byproduct of the site being hosted in the Britain Indian Oceania territory. As such, we settled for the tag agar. If I remember correctly, I was actually one of the voices for going with the name "agar".
Over time, the game has been released for other platforms, including iPhone, android, and even as a facebook application. These platforms do not require a ".io" suffix in the address, but the name has in fact stayed consistent. You can see an example of this in the screenshot from the google store, below:
The same developer eventually released a similar title, slither.io. Whilst involving the same URL suffix, we still tag questions for this game as slither.io.
It does not seem to have caused trouble, in the past. However, we do aim to show consistency, across our tag titles, including both reflecting the actual game name, and showing consistency to any other titles in the series1.
Ultimately, users are still correctly using the agar tag. What I suggest is using agar.io as the primary tag, with agar as a synonym.
1 Whilst some users may not consider the two titles "of the same series", both titles show the exact same presentation in game name and URL, showing argument for the way we interpret the actual name.
