As soon as Applejack showed up in today's episode wearing the fireproof boots, I was like "zomg AJ learned armament haki <_<" because her limbs were black...

anyways, if you know One Piece, you know there's 3 types of haki (special abilities)

Armament haki = body parts turn black and steel to help power physical attacks or block/defend attacks

Observation haki = extrasensory powers and ability to predict opponent movement to avoid/dodge

Conqueror's haki = overtake opponent's willpower and cause them to black out

If the Mane Six were to learn haki, here's who'd get what

Applejack - armament, she's the indestructible earth pony and because she looked like she had it today <_<

Rainbow Dash - observation, due to her speed and agility

Pinkie Pie - observation and armament, her 4th wall breaking abilities man. Her Pinkie Sense and then her body has been through crazy alterations

Fluttershy - conqueror's, just like Coby at the end of the War of Marineford, Fluttershy can tap into a big power bank when she gets pushed to the edge despite her timid, quiet nature

Rarity - observation, she has heightened senses, senses of fashion that is

Twilight - all 3. She's the hero and the main protagonist so duh. She would learn all 3 from Celestia like Luffy learned all 3 from Rayleigh.