A exclamation originally mocking people that don't know that "pwn" is pronounced "own" and not "poan" - hence "ponies". (See definition of "pwn".) Doubly funny now that many of the people who eagerly proclaim "ponies!" are the same people that are being mocked and, yet again, they have no idea what they are saying. Lately, the expression has taken on a life of it's own and is used interchangeably with "PWNED!"
In a video game: "OMGPONIES! Did you see me kill that n00b?!"