A slur typically used against gingers. it originated from an unknown person on Quora saying that ginger hair is just a greasy version of blonde hair. The term pony in it was originally in there to mention longer haired gingers such as Dave Mustaine but has since turned into a generalised slur towards all gingers
Person 1: I love Megadeth but I understand why Metallica kicked him from when he was with them
Person 2: Exactly! You should never trust a Grease Pony!