Myria, I have had three female guinea hogs plus a boar for about two years now. I have one female that gets the skin condition you have mentioned.
I was concerned at first like yourself since it looks so bad. I never really found out what it is but I think its more or less just dry skin and seems to be associated with shedding hair also. She actually has it right now. It should go away in a month or two once she is done shedding and her new hair growth starts. My boar has it also right now just not as bad. There may be a treatment for it but it if there is I havent seeen it.