They seem to be what the Shinra guards in red coats and Russian style hats turn into when they fight
I don't think there's ever really an explanation as to what they are; in fact, there isn't much of an explanation at all about most of the random enemies (as is the case with the majority of RPGs).
Seeing as they are the battle form of the guards, it's probably best to treat them as actual humans standing in a weird position.
Does anything like the grunts ever appear in Crisis Core? I haven't played it for a while, so my memory of most of the enemies is a little hazy. If there are monsters like that in CC, it would probably be best to base your model on them and use slightly stronger colours.