Difference Between Arial Regular and Arial MT?

We recently upgraded to InDesign CC and can no longer find Arial Regular as a font. When I open a document in CC, Arial Regular comes up as a missing font and when I try to substitute from the Style drop-down menu, "Regular" isn't an option anymore (I can see narrow, ital, bold, etc., but no regular). If I get out of the missing font window and just highlight a section of text with the missing font, however, I do see Arial MT as an option in the drop-down menu. What's the difference between Regular and MT? Or perhaps the better question is why doesn't Arial Regular show up in InDesign CC?