A term used to jokingly refer to metal and related genres, particularly more "extreme" forms of metal such as doom metal, black metal and death metal.
The term ponycore is used because fans of these genres feel a need to stop cutting their hair. To keep their long hair from bothering them in day to day life, these people tend to put their hair into a ponytail. The suffix 'core is used because fans of extreme metal tend to look down upon "metalcore", as they consider hardcore punk influenced metal to be inferior to their own types of metal.
Are you listening to ponycore again?
"Yeah man, this band, Black Nephilim of Destruction" is pony to the bone man.