Ya that's the question...WHY do they mimic angels? They are indeed called mimic tangs.
http://www.marinecenter.com/fish/tangs/chocolateyellowmimictang/
http://www.marinecenter.com/fish/tangs/lemonpeelmimictang/
http://www.marinecenter.com/fish/tangs/halfblackmimictang/
http://www.marinecenter.com/fish/tangs/chocolateyellowmimictang/
http://www.marinecenter.com/fish/tangs/lemonpeelmimictang/
http://www.marinecenter.com/fish/tangs/halfblackmimictang/