Longhorn isn’t the official name of the unknown hacking group, it is the name applied to the organization by Symantec researchers.
Is the assigned name “Longhorn” Symantec’s - wink, wink - way of saying they think it is Cult of the Dead Cow but either can’t prove it or don’t want to say it for some reason?
They offer a lot of other breadcrumbs in the April 2017 media stories.