We think he's so hard on Pony because a) he sees Pony's potential and is afraid that if he isn't hard on Pony, then Pony won't reach that potential; and b) he has no experience as a parent! Darry's stepped into this role without any preparation, and is probably really afraid to fail. His little brother's safety is all in his hands, and there's danger lurking around every corner. When Pony doesn't take care of himself or act responsibly, Darry feels frustrated and angry. He has pressure from all sides, and is always on the verge of exploding. According to Pony, Darry wasn't like this before their parents died.
