The official My Little Pony Collectible Card Game has several formats, and the most random is definitely Chaos is Magic. In this format, there is a changing monthly Chaos Rule that you should use as inspiration to design a deck that will do well with that rule. A personal favorite past Chaos Rule was Hearts & Hooves, which let you start the game with two Mane Characters in play. Because, you know, friendshipping.
Anyway, without further ado, here are the Chaos Rules for the fourth quarter of 2016!
October: It's Nightmare Night, Darling! - Cards with the Accessory trait cost exactly 1 action token and have no color requirements. Nothing can reduce or increase the cost or color requirements of cards with the Accessory trait except this rule.
November: Case in Point - Players have the following new action: "Main Phase: spend 1 point to gain 1 Action Token."
December: One Pony's Trash... - Players can play cards in opponent discard piles as if they were in their hand and had no color requirements; when a card played this way would go to the discard pile, banish it instead.